add ReadOnlyProviderError
add ReadOnlyProviderError

 * Some providers are read-only.  If a writing method like upload or
   delete is called on such a provider, it should throw a
   ReadOnlyProviderError, which will return a 501 Not Implemented.

felliott/waterbutler,CenterForOpenScience/waterbutler,RCOSDP/waterbutler,TomBaxter/waterbutler,rdhyee/waterbutler