Error counter

[SijmenHuizenga]

• New: Add a counter for errors that occur during read/write operations.
• Change: Only count bytes as 'written' when the bytes were sent without error

Errors are dropped getting dropped here:

gortsplib/client_media.go

Line 169 in c465264

cm.c.conn.WriteInterleavedFrame(cm.tcpRTPFrame, cm.tcpBuffer) //nolint:errcheck

and we would like to keep track of errors.

We want to track errors because in some cases, when device network connectivity changes, we see situations where gortsplib client keeps a connection still open, while the gortsplib server has already closed the connection. In these situations all packet writes result in an error. Exposing an error count seems like the simplest solution to detect and act on these situations.