Ensure custom exposure mode is supported

package/ios/Core/CameraSession+Video.swift

@@ -200,6 +200,10 @@ extension CameraSession {
           return
         }
 
+        guard captureDevice.isExposureModeSupported(.custom) else {
+          ReactLogger.log(level: .info, message: "Custom exposure mode not supported")
+          return
+        }
         do {
           // Lock the device for configuration
           try captureDevice.lockForConfiguration()