It is common in UEFI code to use construct like this:
```
if (!EFI_ERROR(Status) {
  ...
} else {
  ...
}
```

For the internal implementation of EFI_STATUS functionality look at the files:
MdePkg/Include/Uefi/UefiBaseType.h
MdePkg/Include/Base.h

```
#define EFI_ERROR(A)              RETURN_ERROR(A)

...

/**
  Returns TRUE if a specified RETURN_STATUS code is an error code.

  This function returns TRUE if StatusCode has the high bit set.  Otherwise, FALSE is returned.

  @param  StatusCode    The status code value to evaluate.

  @retval TRUE          The high bit of StatusCode is set.
  @retval FALSE         The high bit of StatusCode is clear.

**/
#define RETURN_ERROR(StatusCode)     (((INTN)(RETURN_STATUS)(StatusCode)) < 0)

...

//
// Status codes common to all execution phases
//
typedef UINTN RETURN_STATUS;
```

So the `EFI_ERROR` macro simply test if a passed value is negative which is the same fact for a signed integer that is has high bit set.

Enumration of possible EFI_STATUS values is in the file MdePkg/Include/Uefi/UefiBaseType.h
```
///
/// Enumeration of EFI_STATUS.
///@{
#define EFI_SUCCESS               RETURN_SUCCESS
#define EFI_LOAD_ERROR            RETURN_LOAD_ERROR
#define EFI_INVALID_PARAMETER     RETURN_INVALID_PARAMETER
#define EFI_UNSUPPORTED           RETURN_UNSUPPORTED
#define EFI_BAD_BUFFER_SIZE       RETURN_BAD_BUFFER_SIZE
#define EFI_BUFFER_TOO_SMALL      RETURN_BUFFER_TOO_SMALL
#define EFI_NOT_READY             RETURN_NOT_READY
#define EFI_DEVICE_ERROR          RETURN_DEVICE_ERROR
#define EFI_WRITE_PROTECTED       RETURN_WRITE_PROTECTED
#define EFI_OUT_OF_RESOURCES      RETURN_OUT_OF_RESOURCES
#define EFI_VOLUME_CORRUPTED      RETURN_VOLUME_CORRUPTED
#define EFI_VOLUME_FULL           RETURN_VOLUME_FULL
#define EFI_NO_MEDIA              RETURN_NO_MEDIA
#define EFI_MEDIA_CHANGED         RETURN_MEDIA_CHANGED
#define EFI_NOT_FOUND             RETURN_NOT_FOUND
#define EFI_ACCESS_DENIED         RETURN_ACCESS_DENIED
#define EFI_NO_RESPONSE           RETURN_NO_RESPONSE
#define EFI_NO_MAPPING            RETURN_NO_MAPPING
#define EFI_TIMEOUT               RETURN_TIMEOUT
#define EFI_NOT_STARTED           RETURN_NOT_STARTED
#define EFI_ALREADY_STARTED       RETURN_ALREADY_STARTED
#define EFI_ABORTED               RETURN_ABORTED
#define EFI_ICMP_ERROR            RETURN_ICMP_ERROR
#define EFI_TFTP_ERROR            RETURN_TFTP_ERROR
#define EFI_PROTOCOL_ERROR        RETURN_PROTOCOL_ERROR
#define EFI_INCOMPATIBLE_VERSION  RETURN_INCOMPATIBLE_VERSION
#define EFI_SECURITY_VIOLATION    RETURN_SECURITY_VIOLATION
#define EFI_CRC_ERROR             RETURN_CRC_ERROR
#define EFI_END_OF_MEDIA          RETURN_END_OF_MEDIA
#define EFI_END_OF_FILE           RETURN_END_OF_FILE
#define EFI_INVALID_LANGUAGE      RETURN_INVALID_LANGUAGE
#define EFI_COMPROMISED_DATA      RETURN_COMPROMISED_DATA
#define EFI_HTTP_ERROR            RETURN_HTTP_ERROR

#define EFI_WARN_UNKNOWN_GLYPH    RETURN_WARN_UNKNOWN_GLYPH
#define EFI_WARN_DELETE_FAILURE   RETURN_WARN_DELETE_FAILURE
#define EFI_WARN_WRITE_FAILURE    RETURN_WARN_WRITE_FAILURE
#define EFI_WARN_BUFFER_TOO_SMALL RETURN_WARN_BUFFER_TOO_SMALL
#define EFI_WARN_STALE_DATA       RETURN_WARN_STALE_DATA
#define EFI_WARN_FILE_SYSTEM      RETURN_WARN_FILE_SYSTEM
```

If you want to see how they encoded look at the MdePkg\Include\Base.h :
```
///
/// The operation completed successfully.
///
#define RETURN_SUCCESS               0

///
/// The image failed to load.
///
#define RETURN_LOAD_ERROR            ENCODE_ERROR (1)

///
/// The parameter was incorrect.
///
#define RETURN_INVALID_PARAMETER     ENCODE_ERROR (2)

<...>

/**
  Produces a RETURN_STATUS code with the highest bit set.

  @param  StatusCode    The status code value to convert into a warning code.
                        StatusCode must be in the range 0x00000000..0x7FFFFFFF.

  @return The value specified by StatusCode with the highest bit set.

**/
#define ENCODE_ERROR(StatusCode)     ((RETURN_STATUS)(MAX_BIT | (StatusCode)))

/**
  Produces a RETURN_STATUS code with the highest bit clear.

  @param  StatusCode    The status code value to convert into a warning code.
                        StatusCode must be in the range 0x00000000..0x7FFFFFFF.

  @return The value specified by StatusCode with the highest bit clear.

**/
#define ENCODE_WARNING(StatusCode)   ((RETURN_STATUS)(StatusCode))
```

Let's use `EFI_ERROR` macro in our code and test error printing with the help of `%r` Print formatting option.

We could test error display with a passing `NULL` pointer instead of ImageHandle:
```
Print(L"________\n");
EFI_GUID **ProtocolGuidArray;
UINTN ArrayCount;
EFI_STATUS Status = gBS->ProtocolsPerHandle(NULL,
                                            &ProtocolGuidArray,
                                            &ArrayCount);

if (!EFI_ERROR(Status)) {
  for (int i=0; i<ArrayCount; i++) {
    Print(L"%g\n", ProtocolGuidArray[i]);
  }
  FreePool(ProtocolGuidArray);
} else {
  Print(L"ProtocolsPerHandle error: %r\n", Status);
}
```

This would give us this message in OVMF:
```
ProtocolsPerHandle error: Invalid Parameter
```