Report meaningful error if parsing a version pragma failed

liblangutil/SemVerHandler.h

@@ -26,6 +26,7 @@
 #include <liblangutil/Token.h>
 
 #include <string>
+#include <optional>
 #include <utility>
 #include <vector>
 
@@ -86,7 +87,9 @@ public:
 	SemVerMatchExpressionParser(std::vector<Token>  _tokens, std::vector<std::string>  _literals):
 		m_tokens(std::move(_tokens)), m_literals(std::move(_literals))
 	{}
-	SemVerMatchExpression parse();
+
+	/// Returns an expression if it was parseable, or nothing otherwise.
+	std::optional<SemVerMatchExpression> parse();
 
 private:
 	void reset();