Add nicer error handling on template compile errors (#21350)

There are repeated issues reported whereby users are unable to interpret
the template errors. This PR adds some (somewhat complex) error handling
to the panic recovery for template renderering but hopefully makes the
interpretation of the error easier.

Reference #21344

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/templates/static.go

@@ -31,6 +31,18 @@ func GlobalModTime(filename string) time.Time {
 	return timeutil.GetExecutableModTime()
 }
 
+// GetAssetFilename returns the filename of the provided asset
+func GetAssetFilename(name string) (string, error) {
+	filename := filepath.Join(setting.CustomPath, name)
+	_, err := os.Stat(filename)
+	if err != nil && !os.IsNotExist(err) {
+		return name, err
+	} else if err == nil {
+		return filename, nil
+	}
+	return "(builtin) " + name, nil
+}
+
 // GetAsset get a special asset, only for chi
 func GetAsset(name string) ([]byte, error) {
 	bs, err := os.ReadFile(filepath.Join(setting.CustomPath, name))