Partial file overwritten/truncated on resume failure using --continue --no-overwrites.

[Warr1024]

Steps to reproduce:

1. Using version eadaf08 of youtube-dl from github.
2. Attempt to resume a partially-downloaded youtube video using --continue --no-overwrites.

Usually, the download resumes correctly without a problem. However, sometimes you will get an error message like:

[download] Resuming download at byte ########
[download] Unable to resume

Actual behavior:

Download is restarted from 0, _truncating_ the partially-downloaded file.

Expected behavor:

Any of the following behaviors would probably be reasonable:

1. Abort the download with an error. The user is left to his/her own devices with the partial file.
2. Wait a short period and retry to resume, up to a certain number of times.
3. Start downloading the file from zero, but do NOT overwrite the original. When the resumed download passes the resume point of the partial file, begin appending new content to the end of the existing file. Input before the resume point can be discarded, or optionally compared with the existing file (an error could be raised on mismatch).

[rhnvrm]

👎 can't reproduce on youtube-dl 2016.07.17

[dstftw]

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