import-checker: track SyntaxErrors

We don't really need to report SyntaxErrors, since in theory
docchecker or a test will catch them, but they happen, and
we can't just have the code crash, so for now, we're reporting
them.

contrib/import-checker.py

@@ -587,12 +587,17 b' def main(argv):'
         localmods[modname] = source_path
     for localmodname, source_path in sorted(localmods.items()):
         for src, modname in sources(source_path, localmodname):
+            try:
             used_imports[modname] = sorted(
-                imported_modules(src, modname, localmods, ignore_nested=True))
+                    imported_modules(src, modname, localmods,
+                                     ignore_nested=True))
             for error, lineno in verify_import_convention(modname, src,
                                                           localmods):
                 any_errors = True
                 print('%s:%d: %s' % (source_path, lineno, error))
+            except SyntaxError as e:
+                print('%s:%d: SyntaxError: %s' %
+                      (source_path, e.lineno, e))
     cycles = find_cycles(used_imports)
     if cycles:
         firstmods = set()