ImageLoaderSVG: Improve error reporting

[akien-mga]

Helps with #68822, which will now report:

WARNING: ImageLoaderSVG: Canvas dimensions of 1045714×1032197 (with scale 1.00) and exceed the max supported dimensions of 16384×16384. The canvas will be cropped.
     at: create_image_from_string (modules/svg/image_loader_svg.cpp:94)
ERROR: ImageLoaderSVG: Couldn't draw ThorVG pictures on canvas.
   at: create_image_from_string (modules/svg/image_loader_svg.cpp:122)
ERROR: Error loading image: res://testingSVG_fromInkscape_0.svg
   at: load_image (core/io/image_loader.cpp:101)
ERROR: Error importing 'res://testingSVG_fromInkscape_0.svg'.
   at: _reimport_file (editor/editor_file_system.cpp:1909)
ERROR: Condition "!importer.is_valid()" is true. Continuing.
   at: reimport_files (editor/editor_file_system.cpp:2123)

It still doesn't mention that the canvas actually doesn't contain drawable elements, but I don't know how to check that with the thorvg API to provide more details. At least it mentions that the drawing step failed.

It also warns about the image getting cropped, which should be an indicator for users that they'll not get the intended result.

There's still a failed reimport loop but that's tracked in #62271, so I think we can consider that this closes #68822.

[fire]

The major structure of the code looks good. I wasn’t able to go into deep review.