API should error when attempting to update a registered meta field holding an invalid value #2838

Based on discussion following #2765:

Registering a meta field defines the data type that field should contain, but there are ways outside of the API's purview that a post could come to hold an invalid value in that meta field, such as a serialized object, which does not match the registered type. If the API tries to update the value of a meta object and the API discovers that an invalid type of data is being stored in that field, it should return an error rather than replacing the value of that field.