Improve Makefile target `makemessages` * Change code to only run msginit, if the file is not there. This avoids losing data if msgmerge fails because of a temorarily bad .po file. Rewrite the used shell code using `if` instead of using more implicit `||` and `&&` operator to make it more readable. * Make the operation stop if one of the commands in the makemessages loop fails, so that errors are propagated up to the build process and can be noticed. Given to upstream at https://github.com/Polyconseil/vue-gettext/issues/84 .

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