Improve error reporting for epl-package-as-description
Catch invalid variable value early

Explicitly signal an error if the variable for
`epl-package-as-description` has an invalid value.  Otherwise
`epl-package-description` triggers the `wrong-type-argument` error which
obfuscates the real culprit in stack traces.
Fix argument type error

We obviously cannot treat a `package-desc` as returned by
`package-buffer-info` as `epl-package`.  Hence, split
`epl-package--package-desc-p` into two functions, one of which does the
actual type test in a safe way, the other does the additional cast, and
use each of these at the proper place.

Regression of #6