Fix IOError when files listed under VCS are missing (#32)

This produces some nonsensical warnings, e.g.

    listing source files under version control: 11 files and directories
    building an sdist: check-manifest-0.22.dev0.tar.gz: 10 files and directories
    some files listed as being under source control are missing:
      LICENCE.rst
    copying source files to a temporary directory
    building a clean sdist: check-manifest-0.22.dev0.tar.gz: 10 files and directories
    files in version control do not match the sdist!
    missing from sdist:
      LICENCE.rst
    suggested MANIFEST.in rules:
      include *.rst

No, stupid computer, adding 'include *.rst' to MANIFEST.in is definitely
not going to help resurrect deleted files.

CHANGES.rst

@@ -5,6 +5,9 @@ Changelog
 0.22 (unreleased)
 -----------------
 
+* Fix IOError when files listed under version control are missing (`issue #32
+  <https://github.com/mgedmin/check-manifest/issues/32>`__).
+
 
 0.21 (2014-06-13)
 -----------------

check_manifest.py

@@ -593,9 +593,14 @@ def check_manifest(source_tree='.', create=False, update=False,
             sdist_files = sorted(normalize_names(strip_sdist_extras(
                 strip_toplevel_name(get_archive_file_list(sdist_filename)))))
             info_continue(": %d files and directories" % len(sdist_files))
+        existing_source_files = filter(os.path.exists, all_source_files)
+        missing_source_files = sorted(set(all_source_files) - set(existing_source_files))
+        if missing_source_files:
+            warning("some files listed as being under source control are missing:\n%s"
+                    % format_list(missing_source_files))
         info_begin("copying source files to a temporary directory")
         with mkdtemp('-sources') as tempsourcedir:
-            copy_files(all_source_files, tempsourcedir)
+            copy_files(existing_source_files, tempsourcedir)
             if os.path.exists('MANIFEST.in') and 'MANIFEST.in' not in source_files:
                 # See https://github.com/mgedmin/check-manifest/issues/7
                 # if do this, we will emit a warning about MANIFEST.in not