better error reason reporting for auto config format

Without this change, the user will not be able to know the detail of error if auto config format detection is used.

config.go

@@ -145,6 +145,7 @@ func loadSingleConfigAutoFormat(input interface{}) (*Config, error) {
 			return nil, newError("config loader not found for: ", extension).AtWarning()
 		}
 	}
+	var errorReasons strings.Builder
 	// no extension, try all loaders
 	for _, f := range configLoaders {
 		if f.Name[0] == FormatAuto {
@@ -153,9 +154,11 @@ func loadSingleConfigAutoFormat(input interface{}) (*Config, error) {
 		c, err := f.Loader(input)
 		if err == nil {
 			return c, nil
+		} else {
+			errorReasons.WriteString(fmt.Sprintf("unable to parse as %v:%v;", f.Name[0], err.Error()))
 		}
 	}
-	return nil, newError("tried all loaders but failed for: ", input).AtWarning()
+	return nil, newError("tried all loaders but failed when attempting to parse: ", input, ";", errorReasons.String()).AtWarning()
 }
 
 func getInputCount(input interface{}) int {