Rollup merge of #145821 - lolbinarycat:compiletest-error-show, r=clubby789

compiletest: if a compiler fails, show its output

Before, when working on something like a `rustdoc-js` test, if you made a syntax error in a rust file, you would not get that error output unless you ran with `--verbose`, which would also cause an enormous amount of other output to be printed as well.  This can also lead to frustration in new contributors who don't think to run with `--verbose`.

Now, if rustc or rustdoc is run by compiletest and produces an non-zero exit code, its output will be printed.

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