The "Pref value" filed is not validated if other fields have invalid values when the experiment is saved #1045

[Affected versions]:

• Firefox Release 65.0.2 and above
• Chrome 72.0.3626.121

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• You are logged with your LDAP account on the "https://experimenter.stage.mozaws.net/" website.
• Have an Pref-Flip Study experiment created on the Experimenter stage website.
• Have a branch created with the "Pref Type" set to "boolean".

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to "https://experimenter.stage.mozaws.net/".
2. Open the created experiment from prerequisites.
3. Click the "Edit" button from "Branches" section.
4. Insert an invalid value in the "Pref value" input field (eg.: 43543).
5. Delete the value from the "Branch Size" input field.
6. Click the "Save Draft" button and observe the behavior.

[Expected result]:

• Error messages are displayed for both input fields.

[Actual result]:

• Only the error message for the input field where the value was deleted (Step 5) is displayed.

[Notes]:

• This issue is not reproducible if all other input fields are completed with valid data. When the "Save Draft" button is clicked the error message is displayed.
• This issue is not reproducible if a invalid JSON value is inserted in the "Pref value" input field.
• Attached is a screen recording of the issue: