Validation of plugin configuration

As an Admin I'd like to see user-friendly validation errors while plugins configuration so that I can figure out what I specified wrong

Right now if we put an invalid value into the plugin, it throws exceptions into console. It is expected that a user-friendly validation is implemented.

Acceptance Tests:

• Given Plugin has configuration fields, these fields should have some validation (at least max boundary)
• Given Admin specified an invalid value into the field and pressed Save, a validation error should appear red
• Given a validation error occurred, configuration changes do not get saved
• Given Admin specified a valid value and pressed Save, configuration gets saved into the DB
• Given Admin chooses Ru or Uk she should see validation errors translated into respective languages.

This is going to be tested in scope of Poulpe Auth Plugin.