auto merge of #10936 : cadencemarseille/rust/issue-10754-std-run-unwrap-on-None, r=alexcrichton

The problem was that std::run::Process::new() was unwrap()ing the result
of std::io::process::Process::new(), which returns None in the case
where the io_error condition is raised to signal failure to start the
process.

Have std::run::Process::new() similarly return an Option\<run::Process\>
to reflect the fact that a subprocess might have failed to start. Update
utility functions run::process_status() and run::process_output() to
return Option\<ProcessExit\> and Option\<ProcessOutput\>, respectively.

Various parts of librustc and librustpkg needed to be updated to reflect
these API changes.

closes #10754

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