[BREAKING] Raise GMTTypeError exception for unsupported image dtypes

[seisman]

GMT only supports images with the uint8 data type. Previously, GMT would emit the following errors for unsupported image data types but would not stop execution:

Error: e [ERROR]: Using this data type (Int8) is not implemented
grdimage (gmtapi_import_image): Bad measurement unit.  Choose among c|i|p [/tmp/pygmt-6rst4f_m.tif]
[Session pygmt-session (1729)]: Error returned from GMT API: GMT_IMAGE_READ_ERROR (22)
[Session pygmt-session (1729)]: Error returned from GMT API: GMT_IMAGE_READ_ERROR (22)

As a result, unsupported images were still read and plotted, although the output was likely incorrect. PyGMT would then issue a warning about the unsupported data type.

It is unclear when this behavior changed, but the GMT dev version now produces the following error instead (xref: https://github.com/GenericMappingTools/pygmt/actions/runs/27249793607/job/80471539938):

Error: e [ERROR]: Using this data type (Int8) is not implemented
pygmt-session (gmtapi_import_image): Bad measurement unit.  Choose among c|i|p [/tmp/pygmt-87amr_xv.tif]
[Session pygmt-session (1729)]: Error returned from GMT API: GMT_IMAGE_READ_ERROR (22)
[Session pygmt-session (1729)]: Error returned from GMT API: GMT_IMAGE_READ_ERROR (22)
Error: e [ERROR]: Option -R parsing failure. Correct syntax:
...
Error: e [ERROR]: Offending option -R0/0/0/0

Apparently, GMT now fails to read images with unsupported data types altogether, causing our tests to fail. I can't find the exact changes that lead to the behavior changes, but I feel it's likely some changes in GDAL.

This PR addresses the issue by raising a GMTTypeError whenever an unsupported image data type is encountered.

[weiji14]

The error only shows up for the GMT dev version right? Not the current stable GMT 6.6.0 release? Ok with raising an error for GMT 6.5 and 6.6 users, since they should really rescale to 0-255 anyway.