fix transaction async error handling that not thrown the full error

[neverlan]

Throw the full error in runTransactionAsync method instead of e.message.

Issue:
If you throw an error in the callback function, after a read for example, the error is misformatted.

Example:

    await db.runTransactionAsync(async (transaction) => {
      try {
        const [[invoice: { status }]] = await transaction.read('Invoices', {
          columns: ['status', 'invoiceId'],
          keys: [invoiceId],
          json: true,
        });
        if (status === "PAID") {
          const error = new Error('Cannot update Invoice as it is already paid');
          error.code = INVOICE_ALREADY_PAID
          error.status = 403;
          throw error;
        }
        // .......
       await transaction.commit();
      }
      catch (err) {
        // console.log(err.code) is undefined
        // console.log(err.status) is undefined
       next(err);
      }

---

Also in version 2.1.0, Transaction.ABORTED is undefined in the error handler (https://github.com/googleapis/nodejs-spanner/blob/master/src/database.ts#L1591), meaning the transaction was never retrieved in case of transient failure. It was fixed in #408. We should publish a new version whenever it's possible.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[neverlan]

I signed it!

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[callmehiphop]

@neverlan I went ahead and added some unit tests for this, if it looks good to you we'll get it merged!

@JustinBeckwith I think I'm going to need some help with the cla

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