ZCFMint.mintGains and ZCFMint.burnLosses should have better error messages

[katelynsills]

The error messages if no arguments are provided is currently:

test(`zcf.makeZCFMint - mintGains`, async t => {
  const { zcf } = await setupZCFTest();
  const zcfMint = await zcf.makeZCFMint('A', MathKind.SET);
  // TODO: create seat if one is not provided
  // https://github.com/Agoric/agoric-sdk/issues/1696
  // @ts-ignore
  t.throws(() => zcfMint.mintGains(), {
    message: 'On demand seat creation not yet implemented',
  });
});

test(`zcf.makeZCFMint - burnLosses`, async t => {
  const { zcf } = await setupZCFTest();
  const zcfMint = await zcf.makeZCFMint('A', MathKind.SET);
  // @ts-ignore
  t.throws(() => zcfMint.burnLosses(), {
    message: "Cannot read property 'getCurrentAllocation' of undefined",
  });
});

We should improve these error messages.