Small fixes to avoid stack traces due to badly formatted .po file

[bryntruscott]

A couple of very small fixes to avoid stack traces due to errors in the .po file.

1. Line 186
   If a keyword - e.g. 'msgid' appears on a line by itself, the 'len(keyword)' call used to raise an IndexError exception rather than reporting the error

2. Line 293
   If the offending line in the .po file contains unicode characters - e.g. accents - the 'format()' call was raising a UnicodeEncodeError rather than reporting the location of the error.

[codecov-io]

Codecov Report

Merging #597 into master will decrease coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
- Coverage   90.33%   90.29%   -0.05%     
==========================================
  Files          24       24              
  Lines        4120     4123       +3     
==========================================
+ Hits         3722     3723       +1     
- Misses        398      400       +2

Impacted Files	Coverage Δ
babel/messages/pofile.py	96.19% <71.42%> (-0.66%)	⬇️

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[akx]

Thank you!