- Bump amcrest package to 1.5.6. Includes networking improvements, no longer
  communicates during Http.__init__(), and allows running snapshot command
  without using stream mode.
- Handle login errors better, and not just at startup.
- Increase network connect & read timeout to 6.05 seconds.
- Increase network read timeout to 20 seconds for snapshot command.
- Run snapshot command in separate task, that cannot be cancelled, to eliminate
  possibility of two snapshot commands running simultaneously (since
  AmcrestCam.async_camera_image can be cancelled.) Also makes sure any exceptions
  from the command are caught properly.

Remove leading underscore from _CannotSnapshot

Add docstring for now public Exception class.