For all the extension entrypoints using the get_buffer() helper, they
wanted the same error handling.  In some cases, the error was doing
the same error return whether target was a bad enum, or a user buffer
wasn't bound.

(Actually, GL_ARB_map_buffer_range doesn't specify the error for a zero
buffer being bound for MapBufferRange, though it does for
FlushMappedBufferRange.  This appears to be an oversight).

Fixes piglit GL_ARB_copy_buffer/negative-bound-zero.

Reviewed-by: Brian Paul <brianp@vmware.com>