Prevent exit codes > 255 in convert.py

[m90]

The colmap executable can return exit codes greater than 255. In the special case of 256, which for example happens on a bad input path

E0523 15:37:08.226591     8 logging.cc:56] [option_manager.cc:833] Check failed: ExistsDir(*image_path)
E0523 15:37:08.226668     8 option_manager.cc:899] Invalid options provided.
ERROR:root:Feature extraction failed with code 256. Exiting.

this seems to overflow to 0, making scripted tools think the invocation succeeded. Considering behavior of POSIX exit codes that are outside of the range between 0 and 255 seems to be undefined, it probably makes sense to just exit 1 on anything above 255. The exit code returned by invoking colmap will still be printed for debugging.