Attach failed message ids to error object

[knicola]

Description: Throw a custom error, when messages fail to send, with an additional property to expose the failed message ids for programmatic access.

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?: In cases where we have to take additional steps for the failed messages, we have to parse the error message in order to get their IDs.

Code changes:

• Add a custom FailedMessagesError error class, with a failedMessages property of type string[] to hold the IDs of the messages that failed to send.

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Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.