Unify and correct error messages

Just a simple cleanup here. I'll leave it open for a couple days before merging in case anyone has comments.

OpenCL error messages are now prefixed correctly with coot::opencl::<function>(): for operations, and runtime initialization errors come with coot::cl_rt::init():. CUDA error messages are coot::cuda::<function>(): and coot::cuda_rt::init():. The idea here is to give the user a basic idea of what component might be having a problem, and to clearly identify it as a Bandicoot problem (which may not be immediately clear to users who are using code for which Bandicoot is a dependency that they may not be working directly with).