Fix OSD crash bug within CUDA & CL compute modules :

- remove the GL error check in cudaGLVertexBuffer :
  * unrelated GL errors left on the stack were triggering erroneous
  vertexBuffer allocation errors
  * we should not be checking for GL errors here anyway (as most other
  buffer allocations aren't checked either)

- add some pointer checking in the GL / D3D drawContexts in case the
  vertexBuffer pointers passed are NULL

- add some additional typedefs in OsdError to report some of the new
  CUDA / GL related errors