Fix error handling for dump symbol graph

[neonichu]

We were ignoring exit codes and output here, making the corresponding tests impossible to debug (and probably also making the command hard to use).

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test

[neonichu]

Ah, this is kinda interesting! Looks like this is already broken on Linux, but because we didn't actually look at errors, we never noticed?

[neonichu]

Actually, seems like it is also broken on macOS.

[neonichu]

Ah yes:

let targets = try buildSystem.getPackageGraph().rootPackages.flatMap{ $0.targets }.filter{ $0.type == .library || $0.type == .executable }

This is wrong of course, because we do not provide any modules for executables unless there's a test target depending on them.

[neonichu]

We'll need to merge #7143 first, so we don't always fail the existing tests.

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test windows