BIND 10 #2518: introduce exception hierarchy for "from text" errors in libdns++

[BIND 10 Development]

#2518: introduce exception hierarchy for "from text" errors in libdns++
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                   Reporter:         |                 Owner:  UnAssigned
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:         |             Sensitive:  0
  libdns++                           |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
  loadzone-ng                        |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 It's better to have some hierarchy in exception classes in libdns++
 so that we can have a base exception class that indicates some
 syntax (and maybe some of semantics) errors in a unified manner.

 My specific suggestion is to define `DNSTextError` in
 lib/dns/exceptions.h, and update existing exception classes thrown
 from "from text" type of methods so that they are derived from
 `DNSTextError`.  Examples are `TooLongLabel`, `BadLabelType`,
 `InvalidRRType`.

 If it's not big, also update the Python wrapper to be consistent with
 that.

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Ticket URL: <http://bind10.isc.org/ticket/2518>
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