Feature: Error or warning when `--pem-file` is `None`.

[matthewhammer]

The error path that I want a warning or explicit error for is I've forgotten to supply the --pem-file option, and the quill subcommand for neuron-manage works, but produces a message that fails on send (bad key).

Instead, I'd like quill to fail early when a nice message when the pem field is missing (None) for any command that uses sign. It avoids a bunch of wasted air-gapped work.