Add ability to localize failed check messages

[bugzilla-to-github]

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Bug 1630979

Bug Reporter: @mathjazz See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1630407

Pontoon uses different libraries to run checks on translations. Errors and warnings produced by these libraries need to be localizable.

Should we choose to localize the messages within Pontoon, we'd also be able to fine tune some of them. See bug 1630407 for an example.