error message for invalid mapping?

I spent the last several days tracking down a bug in my code.  I was
trying to make my pager send a MapItem in response to a page fault, but
the mapping would never take effect.  The page fault would keep
re-occuring.

I finally figured out the problem: I was trying to send an fpage of memory
that I hadn't requested from sigma0.  Since it was part of a "bootloader
specific" region of memory, you have to explicitly request it from sigma0
-- I couldn't just rely on page faults to obtain it.

I was checking the success code from the IPC and I had turned on all
tracepoints.  Neither indicated what the problem was.  Do you think that
this error condition could either:

1. make the IPC fail
2. log a tracepoint message about the failure

I would otherwise like to take this opportunity to propose a patch, but of
all the code in the kernel, I don't think that modifying
generic/linear_ptab_walker.cc is a good first project.  :)

Thanks,
Josh