Better error message for `rbenv shell`

Shell integration is not enabled by default. This means that, from all the
commands from `rbenv commands`, only "shell" won't work right away.

Replace "no such command" with a more descriptive message that points to
`rbenv init` instead.

libexec/rbenv

@@ -106,7 +106,13 @@ case "$command" in
   ;;
 * )
   command_path="$(command -v "rbenv-$command" || true)"
-  [ -n "$command_path" ] || abort "no such command \`$command'"
+  if [ -z "$command_path" ]; then
+    if [ "$command" == "shell" ]; then
+      abort "shell integration not enabled. Run \`rbenv init' for instructions."
+    else
+      abort "no such command \`$command'"
+    fi
+  fi
 
   shift 1
   if [ "$1" = --help ]; then