scripts/maintenance/tag-route53-hosted-zones: Error messages for missing deps

At least on Bash 4.4, the -V form writes a missing-command message to
stderr:

  $ echo $BASH_VERSION
  4.4.23(1)-release
  $ command -v does-not-exist >/dev/null
  $ command -V does-not-exist >/dev/null
  -bash: command: does-not-exist: not found

Because we're not redirecting command's stderr in the script, it will
fall through to the caller's stderr and make it easier for them to
figure out what went wrong.

Also write a "Missing required dependencies" string to stderr.  We'd
had a "Dependencies not installed." string literal from 82bdd9fe
(installer/scripts: AWS tag and delete scripts, 2017-06-28,
coreos/tectonic-installer#1239), but it hadn't been written anywhere
so it was effectively an internal comment.

scripts/maintenance/tag-route53-hosted-zones.sh

@@ -45,8 +45,8 @@ while [ $# -gt 0 ]; do
   shift
 done
 
-if ! command -v jq > /dev/null || ! command -v aws > /dev/null; then
-  "Dependencies not installed."
+if ! command -V jq >/dev/null || ! command -V aws >/dev/null; then
+  echo "Missing required dependencies" >&2
   exit 1
 fi