[error handling] - Required Value Checks
[error handling] - Required Value Checks

This includes the following changes:

- Created a global ui variable.
- Moved constants to the top of the code
- Moved eventRanges variable to higher up in the code
- Moved logChange() up higher in the code
- Created some validation functions that return true if various values
exist in the sheet.
- Added an if statement to various functions. Some validation function
or functions are called before the function runs. If the the data is
valid, then the function runs, if not, the script ends and a UI alert
with more information is triggered.

Change-Id: Ic92037bb9999ab153db753cc7f9508d3f0746320

google/ga4-gtm-migration