Make "Update release branch" workflow fail if npm version fails

[henrymercer]

This fixes a bug in the "Update release branch" workflow that could occur when there are merge conflicts in package.json. These merge conflicts cause the npm version command to fail, but this failure was not causing the workflow itself to fail. As a result, we merged a v2 -> v1 backport that did not update the version number in package.json. The "Tag release and merge back" workflow subsequently did not update the v1 release tag, so the v1 release did not occur.

As a fix, we use subprocess.check_output instead of subprocess.run to check the exit code and fail the workflow if there are merge conflicts bad enough to prevent us from updating the package version.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[aeisenberg]

LGTM, but would be good to add the whitespace change in first.