Smart Start Resumable Uploads

[stephenplusplus]

Some environments don't have writable $HOME directories, which will return an error from gcs-resumable-upload. We should confirm we can write to the directory before attempting a resumable upload.

Here are the ideal scenarios:

file.createWriteStream()

No options.resumable argument supplied.

• Can write to home directory? Start resumable upload.
• Cannot write to home directory? Fall back to simple upload.

We can explain in the docs we will attempt a resumable upload, but won't guarantee it. In either case, the user does not know which was used.

file.createWriteStream({ resumable: true })

Resumable upload was explicitly requested.

• Can write to home directory? Start resumable upload.
• Cannot write to home directory? Emit an error, close the stream.

file.createWriteStream({ resumable: false })

Resumable upload was explicitly denied.

• Do not attempt to write to home directory.
• Start simple upload.