These routines are used for error handling. vpGetError return
the error code from the first function call that failed since the last
call to vpGetError. vpGetErrorString returns a
descriptive string corresponding to an error code. The string is
stored in a global array so it does not need to be copied to separate
memory.
ERRORS
vpGetError cannot fail. A return value not equal to VP_OK means
that some previous function call has failed. vpGetErrorString
returns NULL if code is invalid.