Output exit code in FAILED message

[schultetwin1]

This fixes #1507

Outputs the exit code after "FAILED" for any process that did not
complete successfully.

Output looks like "FAILED (<exitcode>): <outputs>"

[schultetwin1]

I'm not complete happy with the interface of this design (putting the exit code in parens). I'd love to hear other ideas if people have them.

[jonatino]

Any idea when or if this will be merged? It would help a lot when using ninja on CI to have proper sucess/errors. Right now I depend on checking the output to see if the build is there to determine if a ninja subprocess failed or not.

[lazka]

I was just needing this to debug a failing command that didn't have any output and used this rebased on top of the last release (some code has moved, but the basics still apply). Would indeed be nice to have this by default.

[FrancoisBeaune]

Adding a vote to merge this PR, or another one that addresses the problem. It would have been very useful to us for debugging random command failures.

[z2oh]

Just used a custom built ninja with this patch to debug a gnarly compiler crash. Adding my vote to have this feature merged!

[mcprat]

it's a great idea, but I don't think this is the best way to write it