Remove probably unused FEC_SLACK_TOKEN variable

[cnlucas]

We should remove the check for FEC_SLACK_TOKEN in our config file. We are getting an error during builds, but it doesn't look like we are using this variable.

Also re-examine the missing print logic as it is printing out both keys.

[patphongs]

Line 55 prints keys (all required creds) instead of missing ones. So even if just one credential is missing, the error message shows both SQLA_CONN, FEC_SLACK_TOKEN. I suggest updating to print(formatter.format(", ".join(missing)))