Ensure custom exposure mode is supported

package/ios/Core/CameraSession+Video.swift

@@ -200,6 +200,11 @@ 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()