Add error code and description to what() string of exceptions

When building with `CL_HPP_ENABLE_EXCEPTIONS` enabled, exceptions are thrown when OpenCL calls fail. The string returned by the `what()` method of the exceptions tends to only contain the entrypoint name. Adding the error code in numeric and text form would be really useful.

Start by locating the exception classes and their what() implementations in OpenCL-CLHPP, then trace how failures are created when CL_HPP_ENABLE_EXCEPTIONS is enabled. Done means what() includes the error code in both numeric and textual form, alongside the entrypoint name, with behavior checked for a representative failed OpenCL call.

Written by the indexing model from the issue text.