Handle missing Moonlight binary

[wackerl91]

• Luna Version: ~0.7.0
• nVidia GeForce Experience Version: N/A
• Moonlight Embedded Version: N/A
• OSMC Version: N/A

Expected Behaviour:
If the moonlight binary can't be found a dialog should be shown to inform users. Additionally, allow custom locations for the binary (e.g. self-compiled versions).
An exception should not be thrown anymore. All actions that depend on the binary being present should be halted until a binary path is set.

Actual Behaviour:
An exception is triggered, thus the user is not informed properly about the missing binary.
Custom locations aren't possible, only standard locations (/usr/bin and /usr/local/bin) are searched.