Poorly worded error when installing a local snap without assertions

[Evan]

$ snap install cassandra_3.7_amd64.snap
error: cannot find signatures with metadata for snap "cassandra_3.7_amd64.snap"

This error message does a poor job of describing what went wrong and what I can do to remedy the situation.

Might I suggest something like:
$ snap install cassandra_3.7_amd64.snap
Cannot install a local snap without an accompanying local snap-build assertion.
Please provide a snap-build assertion and try again.
Alternatively, re-run with --force-dangerous if you trust the snap content.

[Celso Providelo (cprov)]

Evan,

snap-build doesn't fill the whole trust-path for a snap:

 [snap-build =>] snap-revision => snap-declaration => account-key => account

and since it is either trusted (fully) or not, the clearest thing to do is to suggest `--force-dangerous`.

Other more elaborate plan would be to design a tool for creating and signing all the assertions needed to distribute snaps safely (and pack them please, 5+ files would be annoying).

[Natalia Bidart (nataliabidart)]

Also documentation at snapcraft.io should be updated accordingly.

[XiaoGuo, Liu (liu-xiao-guo)]

I am also having this problems, and all of sudden, I could not install my snap. It would be good to give tips if it fails to install.

[XiaoGuo, Liu (liu-xiao-guo)]

it is also good to have a short-hand option like -d(angerous) so that we do not need to type more words when developing.