Update plugin code style to make it more aligned with other asdf
plugins and the recommendations from Google's Shell Style Guide[1].

Currently, if the asdf installation type is different than "version",
we only print an error message and then continue with the installation
regardless, which might not be desirable.

Thus, change the current behaviour to terminate execution with a non-zero
exit code to inform the user that the installation method is currently
not supported.

The get_download_url() is a helper function that is used to determine
what the artifacts download URL should be for a give combination of an
underlying operating system and its architecture.  At the moment, this
helper functions also includes an error handling code, that aims to
terminate execution should a valid URL couldn't be found.  Similar error
handling code exists in the download script, which is also a caller of
the get_download_url() helper function, however,...