Improve error messages when <!(commands) fail

Make gyp report both the failing command and the gyp file name when a
<!(command) is not successful. Handle non-zero exit status and Popen
exception (usually file not found) error cases.

Patch from tsniatowski@opera.com.

R=scottmg@chromium.org, thakis@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1322643002 .

pylib/gyp/input.py

@@ -893,20 +893,24 @@ def ExpandVariables(input, phase, variables, build_file):
         else:
           # Fix up command with platform specific workarounds.
           contents = FixupPlatformCommand(contents)
-          p = subprocess.Popen(contents, shell=use_shell,
-                               stdout=subprocess.PIPE,
-                               stderr=subprocess.PIPE,
-                               stdin=subprocess.PIPE,
-                               cwd=build_file_dir)
+          try:
+            p = subprocess.Popen(contents, shell=use_shell,
+                                 stdout=subprocess.PIPE,
+                                 stderr=subprocess.PIPE,
+                                 stdin=subprocess.PIPE,
+                                 cwd=build_file_dir)
+          except Exception, e:
+            raise GypError("%s while executing command '%s' in %s" %
+                           (e, contents, build_file))
 
           p_stdout, p_stderr = p.communicate('')
 
           if p.wait() != 0 or p_stderr:
             sys.stderr.write(p_stderr)
             # Simulate check_call behavior, since check_call only exists
             # in python 2.5 and later.
-            raise GypError("Call to '%s' returned exit status %d." %
-                           (contents, p.returncode))
+            raise GypError("Call to '%s' returned exit status %d while in %s." %
+                           (contents, p.returncode, build_file))
           replacement = p_stdout.rstrip()
 
         cached_command_results[cache_key] = replacement

test/errors/error_command.gyp

@@ -0,0 +1,12 @@
+# Copyright (c) 2015 Google Inc. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+{
+  'targets': [
+    {
+      'target_name': 'foo',
+      'type': '<!(["python", "-c", "import sys; sys.exit(3)"])',
+    },
+  ]
+}

test/errors/gyptest-errors.py

@@ -65,4 +65,16 @@
 test.run_gyp('missing_dep.gyp', status=1, stderr=stderr,
              match=TestCmd.match_re)
 
+# Make sure invalid <!() command invocations say what command it was and
+# mention the gyp file name. Use a "random" command name to trigger an ENOENT.
+stderr = (".*invalid-command-name-egtyevNif3.*netDurj9.*missing_command.gyp.*")
+test.run_gyp('missing_command.gyp', status=1, stderr=stderr,
+             match=TestCmd.match_re_dotall)
+
+# Make sure <!() commands that error out result in a message that mentions
+# the command and gyp file name
+stderr = (".*python.*-c.*import sys.*sys.exit.*3.*error_command.gyp.*")
+test.run_gyp('error_command.gyp', status=1, stderr=stderr,
+             match=TestCmd.match_re_dotall)
+
 test.pass_test()

test/errors/missing_command.gyp

@@ -0,0 +1,12 @@
+# Copyright (c) 2015 Google Inc. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+{
+  'targets': [
+    {
+      'target_name': 'foo',
+      'type': '<!(["invalid-command-name-egtyevNif3", "netDurj9"])',
+    },
+  ]
+}