Make @{transformation,validator}_msg more readable

[pinkwah]

Currently, we give badly worded messages when validations fail, such as Is x a string failed for ....

I propose using string.Formatter to generate messages that are easier for humans to parse. For example:

@validator_msg("Expected {name} to be a list, got {type} instead")
def _is_list(x):
  pass

@validator_msg("Expected {name} to be a valid ISO8601(YYYY-mm-dd) date format")
def _is_date(x):
  pass

@validator_msg("Is x an old validator msg")
def _yes(x):
  pass

string.Formatter can check how many fields there are. If 0, then it can format the message as before. Otherwise we can try to subsitute name and type (and maybe others) with relevant data.

[markusdregi]

I completely agree with you! :+1: The change should be populated through our current types, as well as documentation, examples and best practices.