don't try to use db connection if error is thrown

if there are issues connecting to mongo, db.migrationCollection is still called. When db.getConnection fails to get  a connection, db is undefined and this exception is raised:

~/mongo-migrate/index.js:247
var migrationCollection = db.migrationCollection,
			                            ^
TypeError: Cannot read property 'migrationCollection' of undefined

Moving error conditional up, so that an error is logged instead.