Report initialization errors to Sentry from main

[ChimeraCoder]

Summary

Similar to #172, except this doesn't require resolving #133 before merging.

If we encounter fatal errors when initializing Veneur, we should send those to Sentry (if possible), rather than (just) logging them.

Motivation

See #172

Test plan

Tested locally by providing a bad configuration:

$ go build && ./veneur -f ../../example.yaml
INFO[0000] Preparing workers                             number=2
ERRO[0000] Error initializing server                     error=address h::::: too many colons in address
FATA[0010] Could not initialize server                   error=address h::::: too many colons in address

Rollout/monitoring/revert plan

r? @stripe/observability

[aditya-stripe]

r? @stripe/observability

[joshu-stripe]

!

[cory-stripe]

Hey @aditya-stripe, this needs a rebase then we can merge!

[stripe-ci]

Gerald Rule: Copy Observability on Veneur and Unilog pull requests

cc @stripe/observability
cc @stripe/observability-stripe

[cory-stripe]

Hrm, was gonna merge this but getting a test failure on tip that miiiight be transient…