{,cosmosvisor/}Makefile: Add Makefile to cosmovisor/
subdirectory and integrate build target into the
top-directory Makefile.
cosmovisor/:
Create Golang-idiomatic cmd/ subdirectory to make
`cosmosvisor` binary (instead of a binary named `cmd`)
automatically installable with `go get`.
Minor changes to error handling:
- Plain error strings bring more value than the whole
stacktrace in case of dumb configuration errors.
- Output errors to /dev/stderr instead of /dev/stdout.
Remove dependency on github.com/pkg/errors.
Alessio Treglia
Jonathan Gimeno