Handle out-of-range VLANs gracefully

[sajith]

Resolves #356.

[coveralls]

Pull Request Test Coverage Report for Build 11859274966

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 56.93%

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Totals
Change from base Build 11799592589:	0.1%
Covered Lines:	1109
Relevant Lines:	1948

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💛 - Coveralls

[sajith]

@italovalcy The request you've provided in #356 is this:

{
  "name": "VLAN between AMPATH/300 and TENET/300",
  "endpoints": [
    {
      "port_id": "urn:sdx:port:ampath.net:Ampath3:50",
      "vlan": "300"
    },
    {
      "port_id": "urn:sdx:port:tenet.ac.za:Tenet03:50",
      "vlan": "any"
    }
  ]
}

But that does not produce an out of range error (see test status at commit c0389de), so I used 30000 for endpoints.0.vlan instead. The error you reported should be fixed once this PR gets merged. Thank you for the helpful error description!