Better styling (new lines) in plugin validation errors

[vaszig]

The errors of the plugins are correctly retrieved by the backend but form io doesn't treat them as separate errors, so they are all shown as one string.

[sergei-maertens]

Clarification: this is some Form.io cruft in how it collects validation errors, processes them and then feeds them to the EJS template rendering and somewhere in that whole chain the markup gets stripped out (injecting paragraph tags or linebreaks doesn't work).

Disabling template escaping is no option as there may be user input which would allow for XSS.