CLI: 'bin/tahoe @foo ...' gives misleading error message if foo doesn't exist

[davidsarah]

$ bin/tahoe @coverage run --branch @tahoe debug trial

gives this error:

I am a "bin/tahoe" executable who is only for the convenience of running
Tahoe from its source distribution -- I work only when invoked as the "tahoe"
script that lives in the "bin" subdirectory of a Tahoe source code
distribution, and only if you have already run "python setup.py build".

I just tried to invoke my brother at "/home/davidsarah/tahoe/snotty/support/bin/tahoe"
and got an exception.

Traceback (most recent call last):
  File "bin/tahoe", line 116, in <module>
    res = subprocess.call(command, env=os.environ)
  File "/usr/lib/python2.6/subprocess.py", line 470, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.6/subprocess.py", line 623, in __init__
    errread, errwrite)
  File "/usr/lib/python2.6/subprocess.py", line 1141, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

The 'No such file or directory' error is correct, because no 'coverage' script was installed on this system. (The Ubuntu python-coverage package installs it as 'python-coverage' instead.) However the message before that complains about the wrong path, as though it were trying to invoke the support/bin/tahoe script rather than 'coverage'.

[davidsarah]

bin/tahoe-script.template: fix the error message that is displayed when a runner script cannot be found. fixes #1488