Improving error messages from backend to cell popover

Aim of this ticket:

• Improve error messages sent by server to the cell popover in case of specific errors: eg.
  kkufova1@redhat.com: I noticed that Impossible to parse as boolean message keeps getting displayed instead of an actual help. For example, I used falsee instead of false. I think in this case we definitely need The expected value is 'false' but the actual one is 'falsee'. Another scenario: if the expected value is null but I enter [] instead, the error message is Error with this value: Impossible to parse as boolean. It doesn't say anything about the actual value being null.
• Improving exceptions handling related to this part: at the moment, any type of exception could be sent to the popover (eg. OutOfMemory). Only a subset should be managed.

Related to https://issues.jboss.org/browse/DROOLS-3838