frontend: return a user friendly error when modulemd is not ok
Fix #1382
Instead of a typical last-resort exception message
Error: Request wasn't successful, there is probably a bug in the API code
Print something more user-frinedly, e.g.
Error: Unsupported or malformed modulemd yaml - Missing required option
data.summary
Error: Invalid modulemd yaml - Provided YAML did not begin with
a module document
Error: Invalid modulemd yaml - Parser error
Merges: #1384