KeyError if default alias is not present.

[jdn]

I noticed that when I issue the command:

tahoe stats -d storage/

(notice the missing alias) I get this KeyError?:

Traceback (most recent call last):
  File "allmydata-tahoe-1.6.0/support/bin/tahoe", line 8, in <module>
    load_entry_point('allmydata-tahoe==1.6.0', 'console_scripts', 'tahoe')()
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/runner.py", line 102, in run
    rc = runner(sys.argv[1:])
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/runner.py", line 89, in runner
    rc = cli.dispatch[command](so)
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/cli.py", line 488, in stats
    rc = tahoe_manifest.stats(options)
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/tahoe_manifest.py", line 140, in stats
    return StatsGrabber().run(options)
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/slow_operation.py", line 20, in run
    rootcap, path = get_alias(options.aliases, where, DEFAULT_ALIAS)
  File "allmydata-tahoe-1.6.0/src/allmydata/scripts/common.py", line 150, in get_alias
    return aliases[default], path
KeyError: 'tahoe' 

I guess it is because the default alias "tahoe" is not defined, thus I have made the attached patch of src/scripts/common.py. The patch raises an UnknowAliasError? instead of the KeyError?.

149a150,151
>         if default not in aliases:
>             raise UnknownAliasError("Unknown default alias '%s', please create it with 'tahoe add-alias' or 'tahoe create-alias'." % default)

I am not sure that the message is as one would like.

[davidsarah]

Duplicate of #939, which has a more complete fix.