Improve error message for "wheel not found"

[suutari-ai]

Currently, if wheel build fails to produce the wheel file (e.g. because of some build error), a very confusing IndexError is generated. It should rather generate a proper error message with better details of what went wrong.

The IndexError is caused by get_wheels returning an empty list here:

prequ/prequ/scripts/build_wheels.py

Line 51 in ed3b798

built_wheel = get_wheels(conf, pkg, ver)[0]

Thanks to @RauliL and @inesjelovac for reporting this to me.

[suutari-ai]

This might also be caused by misspelling the requirement name in the source requirements so it might be a good idea to add a hint to the error message such as "Is the requirement name spelled correctly?".