[Usability] put the requested device selector string in the exception message

[ogrisel]

At the moment, when a third party library forges an invalid device selector string from some application context (e.g. an environment variable), the user will face the following exception:

  File "dpctl/_sycl_device.pyx", line 293, in dpctl._sycl_device.SyclDevice.__cinit__
dpctl._sycl_device.SyclDeviceCreationError: Could not create a SyclDevice with the selector string

In order to make it easier to find the cause of the problem it would be nice to recall the requested selector string in the error message itself.