[Bug 1800670] [NEW] Improve error reporting to give clearer messages and context

[Mathieu Trudel-Lapierre]

Public bug reported:

Give more information about the error, including its exact location in
the code.

Make the error messages shorter but still understandable.

Present clear context so people understand what we found was wrong, if
not from line/column number, then from the extract from their file, with
a marker to the error.

** Affects: netplan
     Importance: Undecided
         Status: New

** Affects: netplan.io (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: netplan.io (Ubuntu Bionic)
     Importance: Undecided
         Status: New

** Affects: netplan.io (Ubuntu Cosmic)
     Importance: Undecided
         Status: New

** Affects: netplan.io (Ubuntu Disco)
     Importance: Undecided
         Status: New

** Also affects: netplan.io (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: netplan.io (Ubuntu Cosmic)
   Importance: Undecided
       Status: New

** Also affects: netplan.io (Ubuntu Disco)
   Importance: Undecided
       Status: New

** Also affects: netplan.io (Ubuntu Bionic)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1800670

Title:
  Improve error reporting to give clearer messages and context

Status in netplan:
  New
Status in netplan.io package in Ubuntu:
  New
Status in netplan.io source package in Bionic:
  New
Status in netplan.io source package in Cosmic:
  New
Status in netplan.io source package in Disco:
  New

Bug description:
  Give more information about the error, including its exact location in
  the code.

  Make the error messages shorter but still understandable.

  Present clear context so people understand what we found was wrong, if
  not from line/column number, then from the extract from their file,
  with a marker to the error.

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