Do not output invalid UTF-8 for some lexer errors

OMCompiler/Parser/BaseModelica_Lexer.g

@@ -393,9 +393,19 @@ SESCAPE : esc='\\' ('\\' | '"' | '\'' | '?' | 'a' | 'b' | 'f' | 'n' | 'r' | 't'
     char chars[2] = {LA(1),'\0'};
     const char *str = chars;
     int len = strlen((char*)$text->chars);
-    c_add_source_message(NULL,2, ErrorType_syntax, ErrorLevel_warning, "Lexer treating \\ as \\\\, since \\\%s is not a valid Modelica escape sequence.",
+    if ((chars[0] & 0xE0) == 0xD0) {
+      c_add_source_message(NULL,2, ErrorType_syntax, ErrorLevel_warning, "Lexer treating \\ as \\\\, since the next byte is the start of a UTF-8 character and thus not a valid Modelica escape sequence.",
+          &str, 0, $line, $pos+1, $line, $pos+len+1,
+          ModelicaParser_readonly, ModelicaParser_filename_C_testsuiteFriendly);
+    } else if ((chars[0] & 0x80)) {
+      c_add_source_message(NULL,2, ErrorType_syntax, ErrorLevel_warning, "Lexer treating \\ as \\\\, since the next byte is an invalid UTF-8 character and thus not a valid Modelica escape sequence.",
+          &str, 0, $line, $pos+1, $line, $pos+len+1,
+          ModelicaParser_readonly, ModelicaParser_filename_C_testsuiteFriendly);
+    } else {
+      c_add_source_message(NULL,2, ErrorType_syntax, ErrorLevel_warning, "Lexer treating \\ as \\\\, since \\\%s is not a valid Modelica escape sequence.",
           &str, 1, $line, $pos+1, $line, $pos+len+1,
           ModelicaParser_readonly, ModelicaParser_filename_C_testsuiteFriendly);
+    }
   });
 
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