Validate `--bbs-shared-home` and `--archive-path`

[ArinGhazarian]

When running bbs2gh on the Bitbucket instance we need to make sure that if the --bbs-shared-home is provided it points to an existing directory. Additionally if the --archive-path is provided (regardless of where the CLI is running) we need to make sure it points to an existing file.

Currently we don't check those paths and if they are wrong the upload phase errors out but the error message is not user friendly (take this support ticket for example) and can cause some confusion. With this change we can clearly say what is wrong and fail fast.

Todo

• Validate --bbs-shared-home if it's provided and the CLI is running on Bitbucket instance
• Validate --bbs-shared-home if it's provided regardless of where the CLI is running
• Log the full archive path before trying to upload