Make x.py less verbose on failures

[jyn514]

• Don't print the exact command run by rustbuild unless --verbose is set.
  This is almost always unhelpful, since it's just cargo with a lot of
  arguments (and you can't replicate it anyway unless you have the environment variables, which aren't printed by default).
• Don't print "Build completed unsuccessfully" unless --verbose is set.
  You can already tell the build failed by the errors above, and the
  time isn't particularly helpful.
• Don't print the full path to bootstrap. This is useless to everyone,
  even including when working on x.py itself. You can still opt-in to
  this being shown with --verbose, since it will throw an exception.

Before:

error[E0432]: unresolved import `x`
   --> library/std/src/lib.rs:343:5
    |
343 | use x;
    |     ^ no external crate `x`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `std`

To learn more, run the command again with --verbose.
command did not execute successfully: "/home/joshua/rustc4/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "8" "--release" "--features" "panic-unwind backtrace" "--manifest-path" "/home/joshua/rustc4/library/test/Cargo.toml" "--message-format" "json-render-diagnostics"
expected success, got: exit status: 101
failed to run: /home/joshua/rustc4/build/bootstrap/debug/bootstrap check
Build completed unsuccessfully in 0:00:13

After:

error[E0432]: unresolved import `x`
   --> library/std/src/lib.rs:343:5
    |
343 | use x;
    |     ^ no external crate `x`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `std`

To learn more, run the command again with --verbose.

cc #86854, #86022

r? @Mark-Simulacrum

[Mark-Simulacrum]

I'd prefer to keep the time and "unsuccessfully" line - I find it slightly helpful at least and it seems pretty harmless.

Other changes seem great (haven't looked at code yet).

[jyn514]

Sure, done. (I don't find it particularly useful, but we can always remove it later and getting rid of the other three lines seems really helpful.)

[petrochenkov]

Don't print "Build completed unsuccessfully" unless --verbose is set.
You can already tell the build failed by the errors above, and the
time isn't particularly helpful.

I added it exactly because you cannot, and because it's helpful.
x.py failures on CI can happen for numerous reasons (like network failures, timeouts, etc) and the relevant message can be somewhere in the middle of the log.
The "Build completed (un)successfully" line that is always printed allows to see the result immediately in predictable place.
Although some people with good intentions already removed it from x.py --help runs.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit fad1b9c has been approved by Mark-Simulacrum