Navigator.getUserMedia() - Web APIs
if permission is denied, no compatible input devices exist, or any other error condition occurs, the error callback is executed with a mediastreamerror object describing what went wrong.
... }; } errorcallback when the call fails, the function specified in the errorcallback is invokedwith a mediastreamerror object as its sole argument; this object is is modeled on domexception.

MediaStreamTrack.applyConstraints() - Web APIs
if the constraints cannot be applied, the promise is rejected with a mediastreamerror whose name is overconstrainederror, to indicate that the constraints could not be met.

MediaStreamTrack.onoverconstrained - Web APIs
an event handler always has one single parameter, containing the event, here of type mediastreamerrorevent.