On a linker failure, ponyc ought to provide the linker's stderr diagnostics

[slfritchie]

I now stand in the company of several people who've been frustrated by ponyc's lack of diagnostic information when the linking stage fails. Causes such as lack of sufficient virtual memory and missing a required shared library print only Error: unable to link: followed by the linker command & arguments. One could try to run that linker command + args oneself ... but one or more object files have already been deleted by the previous run, so that strategy isn't feasible.

If there's a linker error, I suggest one or both of:

1. provide the contents of linker's stdout and stderr
2. do not remove the .o files created as input to the linker. (Or at least provide a new compiler option to prevent object file removal on error or perhaps unconditionally.)