Enable errcheck linter on production files

[dnephin]

Closes #90

This PR completes the work to enable the errcheck linter on non-test files. There are two changes:

1. handle and print errors related to serving the perfetto backend
2. ignore errors from async calls to conn.send. conn.send captures the error as conn.failure, and any subsequent calls to send will return the first error. Ignoring the send error is these cases should be safe because because the error will be related a bit later, not completely lost.