fix(plugin-window-onerror): Make decorateStack more robust
fix(plugin-window-onerror): Make decorateStack more robust

Only supply values that are of the expected type, as the stacktrace module can throw if given input
of the wrong kind.

Fixes #393.

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js