Add Missing Encoder Check on Startup/Profile load

[derrod]

Description

Adds additional checks to the nvenc test binary, and uses it to show a descriptive error message on startup if NVENC was selected and is now missing.

Example:

Motivation and Context

Want to make it more obvious to users why NVENC has gone missing, especially those with Kepler GPUs.

How Has This Been Tested?

Tested with modifications to the nvenc test binary to produce various error codes.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.