Allow 'a'-lines which don't start with 'a' for backward compatibility.

The removed bridgedb.Bridges.parseORAddressLine() function was able to take a
networkstatus line which started with 'a' or didn't. It's a mess to change all
the code, so for now just log a warning when we recieve a line in
bridgedb.parse.networkstatus.parseALine() which doesn't start with 'a'.
Change logged exception to error in parseALine().
Handle case when (ip, portlist) from parseALine() is (None, None).

 * FIXES a bug where bridgedb.Bridges.parseStatusFile() doesn't handle the
   (None, None) for (ip, portlist) which is returned whenever
   bridgedb.parse.networkstatus.parseALine() couldn't parse the line.
Merge branch 'hotfix/9462B-netstatus-returns-None' into develop