isolate the snippet of source code used in formatted error messages

[db48x]

lest bidi formatting characters cause the error message to be rendered sdrawkcab. In principle we should probably do this to the filename and even the error message, since those could be suspect as well. As ordinary RTL text doesn’t cause any problem, it is in practice not so important. Note also that if flycheck is to be localized, this format string must be localized as well (or the localization system must support such substitutions directly).

Fixes #1919.

[CLAassistant]

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