Continue execution if specfile parsing fails
The unused sources detection feature implemented by class SpecFile
is simply an optimization to avoid downloading unused sources.
The parsing is quite different from other steps performed by rpkg,
which also meant it can fail in new ways.
To avoid situations where an error in this optimization step prevents
usage that would otherwise succeed, this commit changes handling
of such errors from exiting to logging the situation and continuing
with the assumption that all sources in the sources file may
be needed.
Resolves: #583
JIRA: RHELCMP-7087
Merges: https://pagure.io/rpkg/pull-request/581
Signed-off-by: Otto Urpelainen <oturpe@iki.fi>