Bug 34077: Improve documentation and error messages of writeoff_debts.pl

Bug 34077: Improve documentation and error messages of writeoff_debts.pl

* Change the SYNOPSIS to better describe the different ways to use the
  script
* Only show the SYNOPSIS when options used are wrong (unknown option,
  no filter options, or neither -c nor -v)
* Show the options details only with --help
* Clarify the fact that -v is required when -c is not supplied in the
  description of both options
* Print a specific error message for the following cases:
  * no filters options
  * neither -c nor -v was supplied

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>