Allow processing message files that are in locales unknown to Babel

[akx]

⚠️ This slightly changes the API of the Catalog object, so projects doing esoteric things with them (e.g. setting .locale post-fact) may break.

Fixes #553
Fixes #555

[codecov-io]

Codecov Report

Merging #557 into master will increase coverage by 0.02%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master     #557      +/-   ##
==========================================
+ Coverage   90.23%   90.26%   +0.02%     
==========================================
  Files          24       24              
  Lines        4045     4067      +22     
==========================================
+ Hits         3650     3671      +21     
- Misses        395      396       +1

Impacted Files	Coverage Δ
babel/messages/catalog.py	94.67% <96.87%> (+0.05%)	⬆️

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[EndenDragon]

Any updates on this pull request yet?

[akx]

I suppose I'll merge this and we'll see what happens.