Improve handler panic reporting (palantir#145)

Stack traces for handler panics were lost, making it hard to determine
where the panic actually happened. Now, stack traces are captured by a
special error type so that handlers can print them. By default, nothing
actually changes, but I plan to extend the default configuration of
go-baseapp so that these errors are formatted with stack traces in logs.

I copied the error code from my implementation in `bluekeyes/hatpear` so
that they'll share the same interface. The `pkg/errors` package has a
similar interface, but uses package-specific types. Using standard
library types seemed preferrable, even though we have a dependency on
`pkg/errors` already.

Update google/go-github to v43 (palantir#139)

Excavator: Manage go module dependencies (palantir#124)

Excavator: Manage go module dependencies (palantir#120)

Excavator: Manage go module dependencies (palantir#114)

Update dependencies to latest versions (palantir#110)

Update go-github to v39 (palantir#106)

Support remote references in default config (palantir#99)

Update go-github to v38 (palantir#96)

Fix YAMLRemoteRefParser with empty input (palantir#94)

Do not treat empty input as an invalid remote reference, since apps may
use an empty file as an on/off switch. Also add some tests for parsing,
since there are several edge cases now.