This improve the handling of indexes and error messages.

Before this PR all errors reaching an index would be ValueError, thus
not finding a wheel was the same as the index page having a non-handled
content type, or even failing to parse the html because of a typo.

This made it hard to debug, or even realize that an index url was wrong,
or that the pages served by the index were incorrect.

I thus introduce 3 New Errors that do not inherit from `ValueError`:
 - IndexMetadataFetchError
 - UnsupportedParserContentTypeError
 - WheelNotFoundError

The first two of which trigger real error, as they likely suggest a
problem with the index or the user config, and should not be ignored.

In addition the `verbose=` option was unconditionally setting the log
level. I think this is improper as if the logging level is set somewhere
else (to DEBUG, or something else), then it is overwritten.

- This then adds the option to pass `verbose=None`, (and make it the
  default), in which case it will not change the default.

python -m http.server will serve ContentType with `; charset=utf-8`,
which is not recognized.

- This now handle the case where the index ContentType header
  contains parameters for example `; charset=utf-8`, by discarding
  everything after the semicolon `;`; this is not proper parsing, but
  should be sufficient here.

 - I found that more debug logging would be useful and added a number
   of debug logs calls

With this I appear to get proper error message and debug statements
when trying to work on the Cors proxy.

Should close pyodide#123, and pyodide#121, and help with pyodide/pyodide#4898

Co-authored-by: Gyeongjae Choi <[email protected]>