[gdb/cli] Keep track of styling failures in source_cache

[gdb/cli] Keep track of styling failures in source_cache

In source_cache::ensure, keep track of which files failed to be styled, and
don't attempt to style them again in case the file dropped out of the cache.

Tested on x86_64-linux.

Reviewed-By: Lancelot Six <lancelot.six@amd.com>