[utils] Improve swift_build_support.shell failure messages.

 - This improves the error messages when commands fail (or don't exist) to show
   a one-line summary of the issue instead of the Python backtrace, and matches
   what was being done by the matching function in `SwiftBuildSupport`.

Author: ddunbar

utils/swift_build_support/swift_build_support/shell.py

@@ -22,6 +22,8 @@
 import sys
 from contextlib import contextmanager
 
+from . import diagnostics
+
 dry_run = False
 
 
@@ -66,7 +68,16 @@ def call(command, stderr=None, env=None, dry_run=None):
     if env is not None:
         _env = dict(os.environ)
         _env.update(env)
-    subprocess.check_call(command, env=_env, stderr=stderr)
+    try:
+        subprocess.check_call(command, env=_env, stderr=stderr)
+    except subprocess.CalledProcessError as e:
+        diagnostics.fatal(
+            "command terminated with a non-zero exit status " +
+            str(e.returncode) + ", aborting")
+    except OSError as e:
+        diagnostics.fatal(
+            "could not execute '" + quote_command(command) +
+            "': " + e.strerror)
 
 
 @contextmanager

utils/swift_build_support/tests/test_tar.py

@@ -51,7 +51,7 @@ def test_tar_this_file_succeeds(self):
                          expect.format(dest=destination, source=source))
 
     def test_tar_nonexistent_file_raises(self):
-        with self.assertRaises(subprocess.CalledProcessError):
+        with self.assertRaises(SystemExit):
             tar(source='/path/to/something/that/surely/doesnt/exist',
                 destination='/another/path/that/shouldnt/exist')