Better syntax checking of safety pragmas

The syntax checking of safety pragmas is not done before injecting them to UPPAAL. I suggest to improve this as follows:

• Clearly define what is allowed and not allowed (people won't read the UPPAAL documentation)
• Provide in the help a few examples
• Minimum syntax verification: valid block and attributes names
• Verify that the expression is a boolean

Also, in the online help on safety pragmas, we could better separate the images and its associated text (A[] p, etc.), and then provide examples for each case.