remctl_error() returns the error from a previously failed remctl
operation. Its only argument is a remctl struct as returned by
remctl_new(). If the previous remctl operation was successful, returns
the constant string ``No error''.
The caller may not modify the returned string. The string may even be
stored in read-only memory and attempting to modify it may cause a memory
fault. The returned pointer will be invalidated by the next call to any
remctl API function with the same client object, other than
remctl_error().
RETURN VALUE
remctl_error() returns either the previous error or the constant string
``No error''. It will never return NULL.