bzr shows stack trace if BZR_PLUGINS_AT at doesn't have value is specific format

I incorrectly (i.e. without reading the docs) set the value of BZR_PLUGINS_AT to path. This causes bzr to print the stack trace with "bzr: ERROR: exceptions.ValueError: need more than 1 value to unpack". It would be good to have a nicer error message without the stack trace saying that the value should be in "plugin_name@path_to_plugin_source:..." format or something similar.

[grep-release]% echo $BZR_PLUGINS_AT
/home/parthm/tmp/grep-release

[grep-release]% bzr plugins
bzr: ERROR: exceptions.ValueError: need more than 1 value to unpack

Traceback (most recent call last):
  File "/storage/parth/src/bzr.dev/trunk/bzrlib/commands.py", line 911, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/storage/parth/src/bzr.dev/trunk/bzrlib/commands.py", line 1066, in run_bzr
    load_plugins()
  File "/storage/parth/src/bzr.dev/trunk/bzrlib/plugin.py", line 232, in load_plugins
    load_from_path(set_plugins_path(path))
  File "/storage/parth/src/bzr.dev/trunk/bzrlib/plugin.py", line 104, in set_plugins_path
    plugin_name, plugin_path = spec.split('@')
ValueError: need more than 1 value to unpack

You can report this problem to Bazaar's developers by running
    apport-bug /var/crash/bzr.1000.2010-06-08T11:38.crash
if a bug-reporting window does not automatically appear.
[grep-release]%