[clang-format] Improve clang-format-diff.py error message

[DavidSpickett]

Summary

Previously if we couldn't run the clang-format command
for some reason, you'd get an unhelpful error message:

OSError: [Errno 2] No such file or directory

Which doesn't tell you what was happening to cause this.

Catch the error and add the command we were attempting to run:

RuntimeError: Failed to run "<...>/clang-food <...>" - No such file or directory"
RuntimeError: Failed to run "<...>/clang-format <...>" - Permission denied"

[krasimir]

LGTM.

Thank you!

clang/tools/clang-format/clang-format-diff.py

for filename, lines in lines_by_file.items():
     command = [args.binary, filename]
     if args.i:
       command.append('-i')
     if args.sort_includes:
       command.append('-sort-includes')
     command.extend(lines)
     if args.style:
       command.extend(['-style', args.style])
+
+    try:
       p = subprocess.Popen(command,
                            stdout=subprocess.PIPE,
                            stderr=None,
                            stdin=subprocess.PIPE,
                            universal_newlines=True)
+    except OSError as e:
+      # Give the user more context when clang-format isn't
+      # found/isn't executable, etc.
+      raise RuntimeError(
+        'Failed to run "%s" - %s"' % (" ".join(command), e.strerror))
+
     stdout, stderr = p.communicate()
     if p.returncode != 0:
       sys.exit(p.returncode)
 
     if not args.i:
       with open(filename) as f:
         code = f.readlines()
       formatted_code = StringIO(stdout).readlines()