Parser: rename the internal parseStream() method to parseState()

there is still the public variant of Parser::parseStream() which
parses an arbitrary std::istream. the name of the state-taking variant
was just confusing, IMO...
ParserState: make it possible to detect if initialization went wrong

this is required to get meaningful error messages if the file
specified by INCLUDE does not exist.
Merge pull request #348 from andlaus/ParserState_improve_construction_failure_detection

ParserState: make it possible to detect if initialization went wrong