Add user to cache filename; better handle cache load/save failures

[klauer]

Context

Closes #1178

Description

This is my attempt at fixing the issue I reported (#1178) regarding cache files generated by different users on the same system.

Consider this a proposal - I'd be happy to adjust it to your preference.

Three main changes were made:

• Added $USER (via getpass.getuser()) to the cache filename (77d8277)
• Moved the try/except block to include cache load / open(cache_fn) (1c36098)
  • This means that if, for any reason, the cache file fails to load, the normal loading procedure can continue
• Wrapped the cache save section in a try/except block and a similar logger.exception message (835e6923239ee2ecee9a6dc6637a2cd73065e38a)
  • I believe cache save failures should not be fatal

[erezsh]

Thanks, I agree with the changes.

Moved the try/except block to include cache load

The one thing that bugs me is that now it will always say "Loading grammar from cache ...", even when the cache file doesn't exist. At least, we should change it to "checking for cache for grammar", or something of the sort.

@MegaIng Mind also taking a look?

[klauer]

Anything else I can do to help move this along?

[erezsh]

@klauer Sorry, I've been a little busy. I'll try to look into it this weekend.

[erezsh]

Thanks for the PR!