Add Steve Johnson-style parser recovery rules:

SourceUnit = Error $
    Block = '{' Error '}'
    ContractDefinition = '{' Error '}'
    Statement = Error ';'

Co-Authored-By: chriseth <chris@ethereum.org>

liblangutil/ErrorReporter.h

@@ -118,6 +118,9 @@ public:
 		return m_errorCount > 0;
 	}
 
+	// @returns true if the maximum error count has been reached.
+	bool hasExcessiveErrors() const;
+
 private:
 	void error(
 		Error::Type _type,
@@ -149,4 +152,3 @@ private:
 };
 
 }
-