By Pavel Polityuk LVIV, Ukraine (Reuters) – Russian shelling of Mariupol on Thursday prevented a humanitarian convoy reaching the besieged Ukrainian city, local officials said, and dented hopes of evacuating trapped civilians who are increasingly desperate for supplies. Residents have been cowering under fire, and without power or water, in the Black Sea port city of over 400,000 people for more than a week and attempts to arrange a local ceasefire and safe passage out have failed repeatedly. Another “humanitarian corridor” appeared to have failed on Thursday, a day after the bombing of a hosp…