Emit a better error message when a command failed to execute.

[Julian-Wollersberger]

I hit this when 'perl' wasn't installed on my windows machine, when trying to cargo install gitui.
The long and verbose output made it hard to notice the actual error (program not found) hiding in the panic message.
It's still verbose, but the better formatting should improve readability.

Before:

error: failed to run custom build command for `openssl-sys v0.9.72`

Caused by:
  process didn't exit successfully: `D:\Dokumente\Rust\gitui\target\debug\build\openssl-sys-3446ba0d2783aba7\build-script-main` (exit code: 101)
  --- stdout
  [ ... ]

  --- stderr
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: NotFound, message: "program not found" }', C:\Users\Julian\.cargo\registry\src\github.com-1ecc6299db9ec823\openssl-src-111.17.0+1.1.1m\src\lib.rs:477:39
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
error: build failed

After:

error: failed to run custom build command for `openssl-sys v0.9.72`

Caused by:
  process didn't exit successfully: `D:\Dokumente\Rust\gitui\target\release\build\openssl-sys-76722f0f88dd29d1\build-script-main` (exit code: 101)
  --- stdout
  [...]

  --- stderr
  thread 'main' panicked at '


  Error configuring OpenSSL build:
      Command: "perl" "./Configure" "--prefix=D:\\Dokumente\\Rust\\gitui\\target\\release\\build\\openssl-sys-e94b178bbce3b1da\\out\\openssl-build\\install" "no-dso
" "no-shared" "no-ssl3" "no-unit-test" "no-comp" "no-zlib" "no-zlib-dynamic" "no-md2" "no-rc5" "no-weak-ssl-ciphers" "no-camellia" "no-idea" "no-seed" "no-engine" "
no-asm" "VC-WIN64A"
      Failed to execute: program not found


      ', D:\Dokumente\Rust\openssl-src-rs\src\lib.rs:484:9
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
error: build failed

[Julian-Wollersberger]

Thanks!