Strict mode for handling warnings

[lucmoreau]

ProvToolbox is permissive in a number of ways

• prov-n parser is permissive, and recovers from errors parsing attributes in provn #111
• some attributes such as prov:label expects strings, but this is not enforced by the grammar prov label strings and qualified names #104
• qualified names and QNames can be constructed with incorrect syntax http://example.com/?a=b ttl -> provn -> ttl roundtrip error #120
• ...

In some cases, it would be desirable for processing to be stopped when such situations are encountered. In other cases, the current behaviour of printing a warning is fine.

The proposal is to introduce a -strict mode for provconvert, making it return an error code.

[lucmoreau]

An implementation could be to introduce a boolean value "bool fatalWarning=false;" indicating if a fatal warning has been encountered. It then suffices to return an appropriate error code, if this variable was set.

Challenge is to do this in a way that supports parallel executions of the toolbox.