No feedback on construct query errors

[enridaga]

There are cases where the query is malformed but there is no error reporting.

Example:

result = engine.construct(query="constru ct * where { service <x-sparql-anything:> { ?s ?p ?o } }")
print(result)

Shows an empty rdflib graph.

[enridaga]

Probably the solution to #27 would also solve this issue.