Weekly report for week-two.csv

Rows: 4
Total hours (nonblank rows): 11.0

Warnings (0):
  (none)

To regenerate this report, run from the folder containing the script:
  python3 weekly-report.py week-two.csv
This overwrites week-two-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.