error handling: keyword/string validity

[Mouvedia]

Prerequisite:

1. property declarations that are ignored declarations that must be ignored #104 need to produce a comprehensible error message on the log
2. property keywords should be stored Properties shouldn't be reset by any value #103

The last step is to output an error message in the log whenever a keyword or a string (some properties accept a predefined set of strings) is not accepted on the property.
This is not 1. because 1. only checks syntax, nonexistent object names (of the object type(s) accepted) and the value type. A consequence of 2. is that in the case of keywords we will have a different error message (it will also say that we didn't find the object name of type @value).