Create more tests for coverage of thrown JSON errors

[danpape]

There are several places in the code where we are supposed to return/throw an error which don't seem to have any tests created to actually cause that error. For instance, in step 6.8 of generateNodeMap() we throw JsonLdError::ConflictingIndexes and as far as I can tell that error nevers happens through the entire suite of tests we have. We should try to devise a test for that case and submit to the json-ld project. Same for all other errors that may not have any tests.