tx config should check source folder content

[ArtFlag]

Hello,

I would like to suggest to print an error message when the source dir provided to tx config is empty, since it's most likely a user mistake.

tx config mapping-bulk --project documentation --file-extension '.pot' \
   --source-file-dir source/locale/en/ --source-lang en --type PO \
   --expression 'source/locale/<lang>/LC_MESSAGES/{filepath}/{filename}.po'

Would return something like:The source file directory is has not pot files. Are you sure it's the right path?

Thanks!