Weekly report for week-one.csv

Rows: 4
Total hours (nonblank rows): 7.0

Warnings (1):
  - line 4 (V-103): missing hours

To regenerate this report, run from the folder containing the script:
  python3 weekly-report.py week-one.csv
This overwrites week-one-report.txt; the input CSV is never modified.
If any hours value is not a valid non-negative number, the script
exits with an error naming the line and writes no report.