Remove exception throwing from RNG code

[manopapad]

The use of exceptions (and particularly the lumping of all unexpected CUDA errors as CURAND_STATUS_INTERNAL_ERROR) is making it harder to pinpoint the sources of errors.

IMHO further refactoring is warranted for the RNG code. The current code is seemingly trying to use cuRAND statuses exclusively. Most uses of curandStatus_t are simply threading these statuses through the code. This is not necessary for our purely internal use of the cuRAND API. Instead the random module should follow the convention followed in other parts of the code, where "leaf" CUDA library calls are checked for errors directly at the call site (e.g. like CHECK_CUBLAS is used).