Review usage of Reporter::InternalError, addresses BIT-1045.

Replaced some with InternalWarning or InternalAnalyzerError, the later
being a new method which signals the analyzer to not process further
input.  Some usages I just removed if they didn't make sense or clearly
couldn't happen.  Also did some minor refactors of related code while
reviewing/exploring ways to get rid of InternalError usages.

Also, for TCP content file write failures there's a new event:
"contents_file_write_failure".

src/RemoteSerializer.cc

@@ -3584,8 +3584,6 @@ bool SocketComm::ProcessParentMessage()
 			InternalError(fmt("unknown msg type %d", parent_msgtype));
 			return true;
 		}
-
-		InternalError("cannot be reached");
 		}
 
 	case ARGS:
@@ -3609,9 +3607,9 @@ bool SocketComm::ProcessParentMessage()
 
 	default:
 		InternalError("unknown msgstate");
+		return false;
 	}
 
-	InternalError("cannot be reached");
 	return false;
 	}