Add a clear warning if importjs command does not work

[lencioni]

For example, if the importjs command cannot be found, the error is pretty cryptic. Also, if an error happens, like

Error: Failed to load plugin import: Cannot find module 'eslint-plugin-import'

the failure mode is pretty bad.

[trotzig]

I believe I made this better in 273882d. Feel free to reopen if you disagree!