./bootstrap changes

[Damjan Jovanovic <da...@apache.org>]

Hi

I've recently changed main/bootstrap and main/solenv/bin/
download_external_dependencies.pl to fail, exiting with an error code and
printing an error message, if dependencies could not be downloaded from any
available URL.

This is because I observed a large number of build failures on the
buildbots where ./bootstrap failed to download a dependency, and later on
building that dependency or a module relying on that dependency fails.

Additionally, on the buildbots I've made ./bootstrap always run twice,
ignoring failures the first time so that we try to download dependencies
again, with failure the second time aborting the entire build (so we don't
waste time on a build that's doomed anyway). This retrying has already
helped in at least https://ci.apache.org/builders/aoo-win7/builds/181
(which failed later on for other reasons).

Damjan