Moto G2 camera error: "restart camera", or "unable to connect to camera"

When you launch the Motorola Camera app on a Moto G2, and it says "Restart camera" or "Unable to connect to camera", try the following:

• Go to Settings->Apps
• Select the Camera app that has the problem
• Force stop it
• Clear its cache and data
• Restart Camera

If the above did not solve the problem, try the following. Stop when the issue gets fixed:

• Switch off the phone. Gently press the area around the lens and restart the phone.
• Think of any recent change made to the Camera app or any other app that interacts with the Camera e.g. Google Photos. Undo the change. (To see apps in order of installation date: go to Play Store->My Apps->All.)
• If the Camera app was updated recently: uninstall updates to the Camera app and launch it again. If it works, re-install updates and check again.
• If you have Google Photos or a similar app that interacts with your camera, go to its Settings and modify any option that relates to the Camera e.g. checkbox that lets you add a link to Photos in Camera. Save changes. Restore the setting if you prefer. Check Camera now.
• Start Android in Safe Mode and check if the Camera app works. If it does, start third-party apps one by one and check if the Camera works after each step.
• Uninstall updates to the Camera app and launch it again. If it works, re-install updates and check again.
• Go to Settings->Storage. Select "Cached data" and select OK to clear the cache of all apps.
• Visit a service centre as it could be a hardware problem.