Auto merge of #12870 - pathway27:master, r=Wafflespeanut

./mach has a formatting error

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Fix for #12766. Couldn't reproduce the exact error.

I explicitly raised urllib2.URLError to get to the print and then tried using many different variable types for e.reason and src but it still didn't throw a coercion error.

So, as @Wafflespeanut said,
> Instead of appending, we can format the values, which (I'm pretty sure) will fix this error.

which is what i've done.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12766, hopefully.

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests they execute before the test runner is installed. Can't test the bootstraping code.

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