Handle errors and warnings from loadHTML()

Handle errors and warnings from loadHTML()

When parsing invalid documents loadHTML() spits out warnings and errors
which may end up polluting the output of tweeper depending on the value
of the "display_errors" variable in the PHP configuration; this may
result in the output being invalid RSS.

Handling those messages explicitly makes tweeper more robust against
different PHP configurations.

Thanks-to: gregor herrmann <gregoa@debian.org>