Backport of "rtmp2: Improve error messages" into 1.20
Backport of !1403 (merged) "rtmp2: Improve error messages".
Commits:
- 2037f1ed rtmp2/client: Make sure 'code' is not NULL
- a13f234b rtmp2/client: Make sure 'desc' is not NULL
- dcada940 rtmp2/client: Make sure 'reason' is not NULL
- 031d5267 rtmp2/client: Make sure 'salt' is not NULL
- 297cdaa3 rtmp2/connection: Handle EOF like error
- 39bab56a rtmp2/connection: Discern reasons for cancelling all commands
- 9952be2a rtmp2/connection: Pass triggering GError to _emit_error
- f2d7b98e rtmp2/connection: Pass triggering GError in 'error' signal
- b22093be rtmp2: Improve error messages