Added Errors if a source directory is not defined.

[dark-swordsman]

Referencing #833, I had an issue where I didn't specify a source directory and Node's path.resolve() was throwing an error, which was useless information.

For newbies to firebase like me, this should drastically help.

Description

Scenarios Tested

Sample Commands

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[laurenzlong]

Thanks for the contribution! See review comments inline

[laurenzlong]

The second if statement is not necessary, since typeof undefined would return "undefined" and therefore be caught by the first if statement. Could you also revise the error message to:

      throw new FirebaseError(
        chalk.bold("firebase.json") +
          " is missing a " +
          chalk.bold("source") +
          " field in the " +
          chalk.bold("functions") +
          " object. Please add one, and specify the source directory for your functions. For example:\n" +
          chalk.bold('\t"functions": {\n\t\t"source": "functions"\n\t}')
      );

You will also need to add this at the top of the file:
var FirebaseError = require("../../error");

[samtstern]

@dark-swordsman since @laurenzlong requested changes over a year ago I am going to close this as being out of date. If you think you still want to pursue this PR, I would suggest opening a new one since master has moved so much since you started this.

Thank you for the contribution! If you think closing this is the wrong idea, just leave another comment.