Better handling for when user hasn't properly configured Pandora client

[partofthething]

Description:
In the Pandora component, the user is expected to have configured the client to auto-login with their credentials as described in the instructions. However, if the user does not do this, they would have gotten a cryptic error. This PR handles that situation, logs what the problem is, and recovers. If the user configures the client they can try turning on the media player a second time without restarting hass and it will work fine.

Checklist:

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

• Documentation added/updated in home-assistant.io

If code communicates with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.