dc_configure() should return the first error, not the errors of subsequent failing attempts

[r10s]

if dc_configure() fails with the given options (may be none), dc_configure() tries over with some different ports, usernames and/or security options. however, if these attempts also fail, the ui may show the most recent error which may be misleading, see https://github.com/deltachat/deltachat-android/issues/750#issuecomment-468930004

we should at least document this behavior and let it up to the ui to show the first failure only.