More user-friendly API error messages

There is a difference between exceptions that are caused by errors in the code (e.g. passing a non-string to a method that requires a string) and exceptions that are caused by user input. The later should be user-friendly and not contain internal implementation details like variable names.

I also removed the repeated value from one message. It just repeated the value and did not provided additional information.

Example URL: http://www.wikidata.org/w/api.php?action=wbparsevalue&format=jsonfm&parser=decimal&values=1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567