Minor issue with error reporting

[henryiii]

I had a pypoject.toml used for flit and trampolim, and it had a dynamic version. Whey found this, and printed out:

dom_toml.parser.BadConfigError: whey only supports 'classifiers', 'requires-python', and 'dependencies' as dynamic fields.

This doesn't tell me what the offender is, it just lists what it does support (I eventually found it was the version listing). It would be more helpful if this message could look more like this:

dom_toml.parser.BadConfigError: whey only supports 'classifiers', 'requires-python', and 'dependencies' as dynamic fields, not 'version'.

(that is, include the offending item in the message)