By Alan Charlish and Anna Koper WARSAW (Reuters) -Poland failed to reach agreement with air traffic controllers on Friday in a dispute over pay and conditions, a trade union spokeswoman said, moving airlines closer to what European authorities have said could be mass flight cancellations. The disruption, affecting not only flights to and from Poland but also those passing through the country’s airspace, could start on May 1, the day after the end of the notice period for air traffic controllers who chose to quit rather than accept new working regulations they say threaten safety. Talks will re…