Disable device sources when exception occur

Catch exceptions when failing to create a device track source in
WebcamSource and MicrophoneSource components, and disable the component
before re-throwing the exception to get Unity to log it. This ensures
that the components are disabled for consistency.

This is a clean-up loosely related to microsoft#335, though the code triggering
that issue has been changed so the issue does not occur anymore.

libs/unity/library/Runtime/Scripts/Media/MicrophoneSource.cs

@@ -98,10 +98,21 @@ protected async void OnEnable()
             {
                 AutoGainControl = _autoGainControl,
             };
-            Source = await DeviceAudioTrackSource.CreateAsync(initConfig);
-            if (Source == null)
+            try
+            {
+                Source = await DeviceAudioTrackSource.CreateAsync(initConfig);
+                if (Source == null)
+                {
+                    throw new Exception("DeviceAudioTrackSource.CreateAsync() returned a NULL source.");
+                }
+            }
+            catch (Exception ex)
             {
-                throw new Exception("Failed to create microphone audio source.");
+                // Disable MicrophoneSource
+                Debug.LogError("Failed to create audio source for MicrophoneSource component; disabling it.");
+                enabled = false;
+                // Throw again to log the exception message with a callstack
+                throw ex;
             }
 
             IsStreaming = true;

libs/unity/library/Runtime/Scripts/Media/WebcamSource.cs

@@ -316,10 +316,21 @@ protected async void OnEnable()
                 enableMrc = EnableMixedRealityCapture,
                 enableMrcRecordingIndicator = EnableMRCRecordingIndicator
             };
-            Source = await DeviceVideoTrackSource.CreateAsync(deviceConfig);
-            if (Source == null)
+            try
+            {
+                Source = await DeviceVideoTrackSource.CreateAsync(deviceConfig);
+                if (Source == null)
+                {
+                    throw new Exception("DeviceVideoTrackSource.CreateAsync() returned a NULL source.");
+                }
+            }
+            catch (Exception ex)
             {
-                throw new Exception("Failed ot create webcam video source.");
+                // Disable WebcamSource
+                Debug.LogError("Failed to create video source for WebcamSource component; disabling it.");
+                enabled = false;
+                // Throw again to log the exception message with a callstack
+                throw ex;
             }
 
             IsStreaming = true;