Proposed change

Many of the issues raised for the Plex component relate to the media_player.play_media service and (usually silent) failures. Many of these failures are networking setups or lack of client support, but it's always difficult to be certain. This PR adds a few things to improve troubleshooting abilities:

1. Explicit logging when the client does not support playback controls.
2. Logging of the specific media found immediately before playback is requested (in case nothing happens).
3. Use of an improved exception in the plexapi library when media lookup fails.
4. Accept a new media_content_type named "KEY". Instead of the existing escaped JSON required for media_content_id, this allows a user to specify a plain integer associated with the media by the Plex server. This can be found via Plex Web by going to Get Info -> View XML on the media and looking for ratingKey.

Existing style:

entity_id: media_player.plex_player
media_content_type: MUSIC
media_content_id: '{"library_name": "Music", "artist_name": "Stevie Wonder", "album_name": "Innervisions"}'

"Key" style:

entity_id: media_player.plex_player
media_content_type: KEY
media_content_id: 54

I do not intend to document the "key" style at the moment as its use is currently intended for troubleshooting purposes. However, this could become useful in the future if home-assistant/architecture#331 is implemented.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io
  The integration reached or maintains the following Integration Quality Scale:

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