TrajPoca: Fail and warn when point POCA fails on downgrade

Addresses build warnings due to unchecked StatusCode

https://lhcb-nightlies.web.cern.ch/logs/build/nightly/lhcb-head/2558/x86_64-centos7-clang8-opt/LHCb/

Kernel/LHCbAlgs/src/TrajPoca.cpp:229:28: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]	
          restrictRange2 ? minimize( traj1, mu1, restrictRange1, traj2.position( mu2 ), dist, precision )	
                           ^~~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~	
1 warning generated.

status code returned by the downgrade minimize calls are now checked and if they fail this failure is correctly propagated along.