Helper messages for documenting build process

I don't have goimports installed, and running make as described in https://cli.urfave.org/CONTRIBUTING/#development-workflow fails.

./urfave-cli-genflags   
which: no goimports in ...

Took this opportunity to enhance the process a bit.

I see that zz_generated.flags.go is invalid without goimports (for whatever reason it is needed). And I need to check why goimports is not installed into ./.local/bin automatically.

What type of PR is this?

cleanup

What this PR does / why we need it:

Create valid output from make call even if goimports is not installed.

Which issue(s) this PR fixes:

(REQUIRED)

Special notes for your reviewer:

(fill-in or delete this section)

Testing