* Continue using our own error handling logic, and print our problems to stderr.
* Also use the linker's messaging facilities to report a more generic error,
in case stderr doesn't show up for some reason.
Sometimes we wrap exceptions to add more information to what's happening, but
that may end up worse if we don't print out the wrapped exceptions.
At the same time we don't want to flood the user with information if they
didn't ask for it, so only show nested exceptions if they're something we
raised ourselves.
* Move much of ErrorHandler.cs into a partial class in ErrorHandler.tools.cs,
which is referenced by mtouch and mmp (but not our runtime).
* Add ErrorHandler.runtime.cs for runtime-specific bits, including a simpler
version of ErrorHandler.Show. In particular this gets rid of the call to
Environment.Exit, which should never happen at runtime.
* Rename MonoTouchException and MonoMacException to ProductException, which
allows us to remove a lot of ifdefs.
* This required moving Application.LoadSymbols and Target.LoadSymbols to
shared mtouch/mmp code.
Rolf Bjarne Kvinge