fix(levelbuilder): improve error handling in deploy to levelbuilder

[drizco]

while investigating a robo DTL failure where the response logged in slack was from the github api indicating an invalid head property, I realized the error handling in create_pull_request was somehow allowing the execution to get that far, but had to be failing silently earlier, likely in create_branch_from_commit.

create_branch_from_commit does more than just create the branch, it also merges in the last green test commit and pushes that branch to origin. I could see that the branch was created successfully, but I suspect it was never pushed to origin, causing the github api error because it couldn't find the head branch we specified. I think it's possible the merge failed but did not result in any conflicts, which is what was being checked before. in this pr, I added exceptions for specific steps rather than returning a boolean up to the caller to raise a generic exception.

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• Tests provide adequate coverage
• Privacy impacts have been documented
• Security impacts have been documented
• Code is well-commented
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