Add warning result when maxErrors is reached

[tpluscode]

I suppose it would be as simple as a top-level result similar to

[] a sh:ValidationReport ;
   sh:result [
      sh:resultSeverity sh:Warning ;
      sh:resultMessage "The maximum number of error (100) was reached. Not all violations may have been reported" ;
   ] ;
.

Unfortunately, according to paragraph 3.6.2 of the spec, sh:focusNode and sh:sourceConstraintComponent are also required. To conform to this requirement we might add some new term to identify the constraint, like ex:MaxErrorConstraintComponent. I have no (good) idea for the focus node other than a new blank node