setup/runcmd: Properly report errors, if running command is missing

We currently use subprocess.check_output() for running external command
(git), and check_output() checks running command status code, as its
name promises:

    https://github.com/python/cpython/blob/2.7/Lib/subprocess.py#L569

and reports it appropriately to user:

    https://github.com/python/cpython/blob/2.7/Lib/subprocess.py#L411

but if the command is not found, it is just single

    error: [Errno 2] No such file or directory

which gets output to the log and it is not clear what command failed, or
evern if it is from runcmd() call.

Redo our runcmd() manually, so that it always report erros with context
- what command it was trying to run. The error now becomes:

   RuntimeError: (['missing-command'],): [Errno 2] No such file or directory

/cc @Tyagov

setup.py

@@ -19,7 +19,7 @@ from setuptools import setup, Extension, Command, find_packages
 from setuptools.command.build_py import build_py as _build_py
 from setuptools.command.build_ext import build_ext as _build_ext
 from distutils.errors import DistutilsExecError
-from subprocess import check_output as runcmd
+from subprocess import Popen, PIPE
 
 import os
 import sys
@@ -165,6 +165,24 @@ def register_as_entrypoint(func, entryname, groupname, distname):
     assert entryname not in group
     group[entryname] = entrypoint
 
+
+# like subprocess.check_output(), but properly report errors, if e.g. commands is not found
+# check_output(['missing-command']) -> error: [Errno 2] No such file or directory
+# runcmd      (['missing-command']) -> error: ['missing-command']: [Errno 2] No such file or directory
+def runcmd(argv):
+    try:
+        process = Popen(argv, stdout=PIPE)
+    except Exception, e:
+        raise RuntimeError("%s: %s" % (argv, e))
+
+    output, _err = process.communicate()
+    retcode = process.poll()
+    if retcode:
+        raise RuntimeError("%s -> failed (status %s)" % (argv, retcode))
+
+    return output
+
+
 def git_lsfiles(dirname):
     # FIXME dirname is currently ignored
     # XXX non-ascii names, etc