style: update message to a warning to highlight potential misconfiguration

Author: tomowatt

hooks/pre-exit

@@ -42,6 +42,6 @@ if [[ "${builder_remove}" == true ]]; then
     docker buildx stop "${builder_name}"
     docker buildx rm "${builder_name}" ${builder_remove_args:+${builder_remove_args[@]}}
   else
-    echo "~~~ :docker: Cannot remove Builder Instance '${builder_name}' as does not exist"
+    echo "~~~ :warning: Cannot remove Builder Instance '${builder_name}' as does not exist"
   fi
 fi

tests/builder-instances.bats

@@ -183,5 +183,5 @@ teardown() {
     run "$PWD"/hooks/pre-exit
 
     assert_success
-    assert_output "~~~ :docker: Cannot remove Builder Instance 'builder-name' as does not exist"
+    assert_output "~~~ :warning: Cannot remove Builder Instance 'builder-name' as does not exist"
 }