Improve error message for type constraints

It was missing a space between the different types, when there
were 3 possible type values.

Signed-off-by: Mazz Mosley <[email protected]>

compose/config/validation.py

@@ -136,7 +136,7 @@ def process_errors(errors, service_name=None):
             if len(validator) >= 2:
                 first_type = anglicize_validator(validator[0])
                 last_type = anglicize_validator(validator[-1])
-                types_from_validator = "{}{}".format(first_type, ", ".join(validator[1:-1]))
+                types_from_validator = "{}".format(", ".join([first_type] + validator[1:-1]))
 
                 msg = "{} or {}".format(
                     types_from_validator,
@@ -156,7 +156,6 @@ def process_errors(errors, service_name=None):
         Inspecting the context value of a ValidationError gives us information about
         which sub schema failed and which kind of error it is.
         """
-
         required = [context for context in error.context if context.validator == 'required']
         if required:
             return required[0].message