handle invalid translation file format

[woru]

expected one element but was: <Return, Return, Return, Return>
java.lang.IllegalArgumentException: expected one element but was: <Return, Return, Return, Return>
at com.google.common.collect.Iterators.getOnlyElement(Iterators.java:312)
at com.google.common.collect.Iterables.getOnlyElement(Iterables.java:289)
at com.github.letsdrink.intellijplugin.translation.TranslationParser.parse(TranslationParser.java:26)