Propagate regex compile failures out of filter creation.
Make calls to owl_filter_init_fromstring fail if the filter includes an
illegal regex (e.g. an initial *), by having owl_filterelement_create_re
return an error code which propagates up.
This fixes an issue where a command like 'unpunt class *' would segfault
because it succeeds in creating the invalid filter, but then fails to
print it successfully.
fixes: #70