fix: non-json config in homedir

[alexeldeib]

Fixes #14

Adds err check on file read for config initialization.

[t0mk]

thanks for the PR @alexeldeib, looksgood!

[jasmingacic]

This fix is breaking the CLI each command that is ran without a config file results in this:

packet plan get 
Unable to read in config file, check the file path and defaults (./packet-cli.{yaml,json} in user home directory) and try again.

It seems even when config file is not present it overrides the environment variable.

[jasmingacic]

Also the message is incorrect. The file created should be .packet-cli.{yaml,json} not ./packet-cli.{yaml,json}
This does not reflect what users are supposed to do according to the docs https://github.com/packethost/packet-cli#authentication