Closes #25.

* Wrote Fetch.ErrorHandling.runExceptT' as a wrapper for runExcepT. It
prints all errors before returning the result of running an Exception
monad.
* Changed the return type of mapErr and mapErr from ErrorIO
[NetworkError] to ErrorIO (). The list of errors is thrown instead of
returned. Thereby, they harmonize with runExceptT's automatic error
printing.
* Fetch.ErrorHandling.printError now prints to stderr. * Wrote
Fetch.ErrorHandling.printErrors.