non existent auth causes traceback

[jmrodri]

if we try to add/edit a profile and give it an auth that doesn't exist, we
get an exception, and then the config is busted.
Profile f11-bogus-multiauth-multiport edited
[adrian@alikins rho]$ rho dumpconfig
Traceback (most recent call last):
File "/home/adrian/src/rho/bin/rho", line 22, in
CLI().main()
File "/home/adrian/src/rho/src/rho/cli.py", line 95, in main
cmd.main()
File "/home/adrian/src/rho/src/rho/clicommands.py", line 133, in main
self.config = self._read_config(self.options.config, self.passphrase)
File "/home/adrian/src/rho/src/rho/clicommands.py", line 98, in _read_config
return config.ConfigBuilder().build_config(confstr)
File "/home/adrian/src/rho/src/rho/config.py", line 289, in build_config
config = Config(auths=creds, profiles=profiles)
File "/home/adrian/src/rho/src/rho/config.py", line 93, in init
self.add_profile(profile)
File "/home/adrian/src/rho/src/rho/config.py", line 134, in add_profile
c)
rho.config.ConfigError: No such credentials: rsa_key