More verbose error message for redis connection failure (#479)

[lkatalin]

This PR adds a bit more context to the panic that results when Rekor can't connect to the redis server. It will look like this:

2021-11-03T16:07:17.091-0400	PANIC	api/api.go:165	failure connecting to redis instance: 
dial tcp 127.0.0.1:6379: connect: connection refused

rather than

2021-11-03T12:54:43.331-0400	PANIC	api/api.go:165	dial tcp 127.0.0.1:6379: connect: 
connection refused

Closes #479