[CDF-25263] 💣 Bom character (#1765)

# Description

See below

## Changelog

- [x] Patch
- [ ] Minor
- [ ] Skip

## cdf

### Fixed

- [alpha] Running `cdf migrate timeseries --mapping-file my_file.csv`
handles BOM character at the beginning of the csv file.

### Improved

- [alpha] Running `cdf migrate timeseries --mapping-file my_file.csv`
has more informative error messages.

## templates

No changes.
