Provide app's appdata problems in the api

[jgarciao]

UpdateServiceImpl print in the logs the appdata problems found in some apps by AppdataValidator, like missing icons, etc.

It would be useful add this info in the api, so it could be shown in a specialized page in the website to help developers fix the problems with their apps

Related to #21

[nedrichards]

This is probably obsoleted by support for the validator in the build process.

[barthalion]

Not exactly. FontForge failed to display because had too long and too many per-language <p> descriptions and we still need to add check for icon availability in buildbot (svg/png with correct sizes).