cosmovisor: various small improvements

[alessio]

{,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.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #7275 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7275   +/-   ##
=======================================
  Coverage   55.15%   55.15%           
=======================================
  Files         584      584           
  Lines       40504    40504           
=======================================
  Hits        22340    22340           
  Misses      16290    16290           
  Partials     1874     1874           

[tac0turtle]

utACK, one less dep 😄