Signature Validation : Change Error Thrown

This commit changes the error reported when the signature
requirements are not met. Instead of reporting "InternalError",
the newly defined InvalidSignature error is thrown in its place.
This allows there to be a more specific error shown.

Tested:
Before:
Apr 08 19:04:44 witherspoon phosphor-image-updater[416]: Error occurred during image validation
Apr 08 19:04:44 witherspoon phosphor-image-updater[416]: The operation failed internally.

After:
Jan 01 00:11:10 witherspoon phosphor-image-updater[403]: Signature Validation failed for image version.

Signed-off-by: Zami Seck <zimzam17@gmail.com>
Change-Id: Ia92aba70ea4b346b1473b557a5adeeea7a3cae3d
1 file changed