Throw proper error message for configuration type mismatch

[jennyzhang-petuum]

Is your feature request related to a problem? Please describe.
When running SpacyProcessor, the config field processor need to be a list of string and it didn't throw error when processor field was provided a string but this string would not work as this config field. Because it does an iteration of the config in the set up part, if the config is a string then it simply loop each character.

Describe the solution you'd like
To do a check on the Type of the config field processor when read in. It could be done in the function validate_spacy_configs