Enhance gradle config annotation processor error report when inconsistencies in ConfigData annotation

[mxtartaglia-sl]

Annotation processor code is not clear about the specific error when somebody mistakenly creates an annotation that sets the value (interpreted as the name of the property to read) instead of defaultValue (interpreted as the value to set the property to when it was not indicated in the file).
We aim to let users know what the error is and how to fix it so it does not require a debugging session for realizing what the problem is.