Add more details to exception when failing for invalid tree.mode

[robaiken]

What are you trying to accomplish?

We are trying to debug and issue where Dependabot fails to create a pull request with the error message "422 - Must supply a valid tree.mode". Increase logging will help us understand the issue better.

How will you know you've accomplished your goal?

We will get details about the file that is causing the issue and the backtrace of the error.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.