‘Rocket Man’, ‘Your Song’, ‘Tiny Dancer’, ‘Goodbye Yellow Brick Road’, ‘I’m Still Standing’... Bernie Taupin has written the lyrics for so many Elton John classics he can barely remember half of them. That may have something to do with his boozy Caribbean escapades in the 1970s — and the addiction support he was giving his lifelong friend and songwriting partner, he tells Craig McLean