Improve messages in custom validators

[jmartinm]

Custom validators are defined https://github.com/inspirehep/record-editor/blob/master/src/app/shared/services/app-config.service.ts#L510-L512

If, for example a date is wrong, the message: "Should match format 'date'" is displayed, which is not useful to know which formats are allowed.

An improvement would be to able to customize these messages.

[harunurhan]

Blocked by this inveniosoftware-contrib/ng2-json-editor#417

[harunurhan]

this is was unblocked while ago, I just realized. If you post list of validations error messages to configure and what to put instead of the default one, I'll add them to configurations.
@jmartinm @StellaCh