Tools telemetry: accurately report when feature flags were inaccessible

[angelapwen]

Merge / deployment checklist

Previously, we sent a false value of tools_feature_flags_valid in our status reports when we were actually unable to access our feature flags, for example in cases where the repository does not have the appropriate security permissions to access feature flags. This change should allow us to catch the error where we are unable to access feature flags, and not send the tools_feature_flags_valid field at all to the status report.

This will enable us to create a monitor on the tools_feature_flags_valid telemetry field, as we will always expect it to be 0.

• 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.

[henrymercer]

Looks good, just a couple of minor comments.