Introduce ff_http_averror()

int ff_http_averror(int status_code, int default_averror)

This helper function returns AVERROR_ value from 3-digit HTTP status
code.

Second argument, default_averror, is used if no specific AVERROR_ is
available. It is introduced because in different places of code
different return codes are used - -1, AVERROR(EIO), AVERROR_INVALIDDATA.
Use ff_http_averror()
Introduce ff_rtsp_averror()

Currently this is another name for ff_http_averror()
Use ff_rtsp_averror()