Replace assert with Mapping Error Exception.
Replace assert with Mapping Error Exception.

If the native path doesn't equal the given path, throw a mapping exception
instead of just asserting. This allows ignoring this error if we want to
ignore mapping errors (which it is).

TBR=maruel@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/12377078

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@185899 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py