pkg(mono-6.12.0): redirect mono error to stderr

packages/mono/6.12.0/compile

@@ -1,13 +1,20 @@
 #!/bin/bash
 
+check_errors () {
+    grep -q 'error [A-Z]\+[0-9]\+:' check.txt && cat check.txt 1>&2 || cat check.txt
+    rm check.txt
+}
+
 case "${PISTON_LANGUAGE}" in
     csharp)
         rename 's/$/\.cs/' "$@" # Add .cs extension
-        csc -out:out *.cs
+        csc -out:out *.cs > check.txt
+        check_errors
         ;;
     basic)
         rename 's/$/\.vb/' "$@" # Add .vb extension
-        vbnc -out:out *.vb
+        vbnc -out:out *.vb > check.txt
+        check_errors
         ;;
     *)
         echo "How did you get here? (${PISTON_LANGUAGE})"