Launch an UnexpectedError when grammar contains issues

Number of grammar issues were previously output. This was useful for
early language developments but always displaying information is
irrelevant.

Note grammar issues means that the grammar itself is buggy. It is not
related to errors in user scripts.

src/language/PugsParser.cpp

@@ -37,7 +37,11 @@ parser(const std::string& filename)
 {
   const size_t grammar_issues = analyze<language::grammar>();
 
-  std::cout << rang::fgB::yellow << "grammar_issues=" << rang::fg::reset << grammar_issues << '\n';
+  if (grammar_issues != 0) {
+    std::ostringstream os;
+    os << "invalid grammar: " << rang::fgB::yellow << grammar_issues << rang::fg::reset << " were detected!";
+    throw UnexpectedError(os.str());
+  }
 
   std::cout << rang::style::bold << "Parsing file " << rang::style::reset << rang::style::underline << filename
             << rang::style::reset << " ...\n";