Fix uninformative error message

[abravalheri]

It seems that there was an easy oversight when implementing the error message for invalid dynamic fields.

Right now, it shows the name of all fields that are listed (including the valid ones), instead of showing only the name of the invalid field.

For example, exception looks like the following:

packaging.metadata.InvalidMetadata: ['author', 'author-email', 'classifier', 'description', 'description-content-type', 'download-url', 'home-page', 'keywords', 'license', 'license-file', 'maintainer', 'maintainer-email', 'obsoletes', 'platform', 'project-url', 'provides', 'provides-extra', 'requires', 'requires-dist', 'requires-python', 'summary'] is not a valid dynamic field

... which is not very great, because it lets to the user the work of figuring out which one of these fields is invalid..

A better error message (which I believe to be the original intent) would be:

packaging.metadata.InvalidMetadata: 'license-file' is not a valid dynamic field