core: Display invalid type in error message

If a variable type which is invalid (e.g. "stringg") is declared, we now
include the invalid type description in the error message to make it
easier to track down the source of the error in the source file.

config/config.go

@@ -807,7 +807,7 @@ func (v *Variable) ValidateTypeAndDefault() error {
 	// If an explicit type is declared, ensure it is valid
 	if v.DeclaredType != "" {
 		if _, ok := typeStringMap[v.DeclaredType]; !ok {
-			return fmt.Errorf("Variable '%s' must be of type string or map", v.Name)
+			return fmt.Errorf("Variable '%s' must be of type string or map - '%s' is not a valid type", v.Name, v.DeclaredType)
 		}
 	}