[#148] Add try/except block around code adding labels to already existing issues

[calebevans]

We have noticed when adding a new label to an already existing issue in Jira that the permissions on the firewatch-tool user needed to be updated in our projects. This PR adds some logging around this issue to help users who may not be familiar with the problem. It also adds some specific documentation to the README to help resolve the problem or tell new users what is required before using the tool.

Closes #148

[redhat-qe-bot]

Report bugs in Issues

The following are automatically added:

• Add reviewers from OWNER file (in the root of the repository) under reviewers section.
• Set PR size label.
• New issue is created for the PR. (Closed when PR is merged/closed)
• Run pre-commit if .pre-commit-config.yaml exists in the repo.

Available user actions:

• To mark PR as WIP comment /wip to the PR, To remove it from the PR comment /wip cancel to the PR.
• To block merging of PR comment /hold, To un-block merging of PR comment /hold cancel.
• To mark PR as verified comment /verified to the PR, to un-verify comment /verified cancel to the PR.
  verified label removed on each new commit push.
• To cherry pick a merged PR comment /cherry-pick <target branch to cherry-pick to> in the PR.
  • Multiple target branches can be cherry-picked, separated by spaces. (/cherry-pick branch1 branch2)
  • Cherry-pick will be started when PR is merged
• To build and push container image command /build-and-push-container in the PR (tag will be the PR number).
• To add a label by comment use /<label name>, to remove, use /<label name> cancel

Supported /retest check runs

Supported labels

• hold
• verified
• wip
• lgtm

[calebevans]

/verified