`multipass find` does not honour "remote" arg with blueprints

[ricab]

There is a certain inconsistency in the handling of remotes in find:

$ multipass find nonsense:charm-dev
Image                       Aliases           Version          Description
charm-dev                                     latest           A development and testing environment for charmers

But

$ multipass find nonsense:
find failed: Remote 'nonsense' is not found. Please use `multipass find` for supported remotes and images.

We should error out in the same way in the first case. Alternatively we could update docs to reflect the "remote" part being ignored with blueprints, but I think that's a lesser solution since we don't know if the argument is meant as a blueprint.