Use err instead of ex in catch

Author: AdamStone

src/core/components/pin.js

@@ -66,7 +66,7 @@ module.exports = function pin (self) {
       update: (multihash, cb) => {
         try {
           multihashes.validate(multihash)
-        } catch (ex) { return cb(ex) }
+        } catch (err) { return cb(err) }
 
         normalized.hashes.push(multihash)
         cb()