Changed handling of switchContext(WEBVIEW_ID) by normalising the input.

Now, commands to switch to "WEBVIEW" will be interpreted as
"WEBVIEW_0".
Also, switchContext(INVALID_HANDLE) will throw an exception.

Fixes #362

NOTE: In case of INVALID_HANDLE the exception thrown is "NoSuchWindow".
In the future this will have to change to "NoSuchContext".