Export MessageNotFoundErr

[bep]

So a client can react to that particular error situation.

[codecov]

Codecov Report

Merging #124 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master     #124   +/-   ##
=======================================
  Coverage   67.76%   67.76%           
=======================================
  Files          28       28           
  Lines        2286     2286           
=======================================
  Hits         1549     1549           
  Misses        659      659           
  Partials       78       78

Impacted Files	Coverage Δ
v2/i18n/localizer.go	81.81% <50%> (ø)	⬆️

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[bep]

Note that this comes from #125 -- which I guess could replace this PR if that worked. But I think it would be cleaner to just check the error.

Also, I don't understand the "codecov error". You may want to add something ala this: https://github.com/bep/s3deploy/blob/master/codecov.yml

[nicksnyder]

I don't have a problem with this PR but it would be helpful to me (for my own understanding) if you could explain your use-case for explicitly wanting to check for this error.

[bep]

If a message with a given ID in a given language does not exist, I want to be able to both

1. Print a WARNING that there is a missing translation
2. Print the default language's version as a fallback

[nicksnyder]

Do you still think this is useful to export?

[bep]

Do you still think this is useful to export?

Yes, as the API is now, these are the cases:

1. Found in the current language
2. Not found in the current language, but in some other language (check language tag)
3. Not found in any language (check error type)
4. Another error ... fail.

The API above could maybe be improved, but I cannot see how 3) can be handled without a way to check the error type (something ala os.IsNotExist).