Do not build/load supported languages with no translation files

When the user specified a language as a supported language
in his project-tap.i18n, we used to create a unified translation
file for that language even if there were no translation files for
that language (even if it was basically an empty JSON).

Later when a client set that language as the client language we
loaded that file even though it is empty.

From now on, no unified translation file will be created for
supported languages that have no translation files and we
won't attempt to load them.
[MINOR] readme improvements
build plugin: catch all build errors and use compileStep.error to report them
[MINOR] readme updates
Refactor build plugin. Fix a rare bug causing the proj base lang not to load
Bump version to v0.7.0