sipvoiplink: don't throw if resolver creation fails

Using pjsip' resolver is optional: pjsip fallbacks
to gethostbyname():
http://www.pjsip.org/pjsip/docs/html/group__PJSIP__ENDPT.htm#gaa783363fa5b0d6d1fd081bfd40d4314b
Also the DNS resolver is only useful for SIP.

If pj_dns_resolver creation fails for some reason,
print an error message and continue, instead of
preventing the SIPVoIPLink instance to be created and
prevent proper initialization of Ring.

Change-Id: I92e27d53f3fd094322e923a9e418de6cda14b945