fix(screenshare): stop using exact for specified device sharing (#1031)

Overconstrainederror occurs when calling gum with "exact"
when on URLs with a hash. To get around this, remove
"exact". Add handling for specified device not being
found, as we can no longer rely on "exact". This
still leaves open the possibility of the wrong
camera being accessed.

modules/RTC/RTCUtils.js

                         device.kind === 'videoinput'
                             && (device.deviceId === desktopSharingSourceDevice
                             || device.label === desktopSharingSourceDevice));
+
+                if (!matchingDevice) {
+                    return Promise.reject(new JitsiTrackError(
+                        { name: 'ConstraintNotSatisfiedError' },
+                        {},
+                        [ desktopSharingSourceDevice ]
+                    ));
+                }
+
                 const requestedDevices = [ 'video' ];
 
                 // Leverage the helper used by {@link _newGetDesktopMedia} to
                 const { gumOptions, trackOptions }
                     = this._parseDesktopSharingOptions(options);
 
-                // Create a custom constraints object to use exact device
-                // matching to make sure there is no fallthrough to another
-                // camera device. If a matching device could not be found, try
-                // anyways and let the caller handle errors.
                 const constraints = {
                     video: {
                         ...gumOptions,
-                        deviceId: {
-                            exact: (matchingDevice && matchingDevice.deviceId)
-                                || desktopSharingSourceDevice
-                        }
+                        deviceId: matchingDevice.deviceId
                     }
                 };