Add an onerror handler to shutil.rmtree calls in tick_my_feedstocks.py

[tkelman]

Should avoid errors such as
PermissionError: [WinError 5] Access is denied: 'C:\\Users\\akelman\\AppData\\Local\\Temp\\tmpu3fwwzf4\\botocore-feedstock\\.git\\objects\\pack\\pack-75b2092341bff03adcd15b32b752762b5801440f.idx'

[tkelman]

I'd also kind of like to move the conda_smithy imports inside the regenerate_fork function so it's an optional dependency that you can avoid if you use --no-regenerate, but let me know if anyone has any preference for doing that in this PR or a separate one.

[pmlandwehr]

Thanks for this - might also want to check how it squares with the develop branch of the other tick-my-feedstocks I should really seem about merging those changes back into here at some point...

[tkelman]

Right, saw that, opened pmlandwehr/tick-my-feedstocks#19 with basically the same change.

For another couple-line but only partially related change, would you prefer it to be added to these existing PR's or done separately?