Improve error message for gt functions

[olivroy]

Summary

Use rlang::caller_env() to help user find the source of the error more easily.

# before 
mtcars |> fmt_number()
Error in `stop_if_not_gt_tbl()`:
! The `data` provided is not a `gt_tbl` object.

# With this PR
mtcars |> fmt_number()
Error in `fmt_number()`:
! `data` must be a `gt_tbl` object, not a data frame.

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[rich-iannone]

This is excellent. Thank you!