Stricter connection field check and error codes

[YufengXin]

To close the following issues:

• End-to-end test fails when trying to create a L2VPN with 'max_delay' out of range #365
• End-to-end test fails when trying to create a L2VPN with 'max_number_oxps' out of range #366
• End-to-end test fails when trying to create a L2VPN with impossible scheduling #367
• End-to-end test fails when trying to create a L2VPN with incorrect body #362
• End-to-end test fails when trying to edit a L2VPN with incorrect body #369
• End-to-end test fails when trying to edit a L2VPN with max_delay out of range #372
• End-to-end test fails when trying to edit a L2VPN with max_number_oxps out of range #373
• End-to-end test fails when trying to edit a L2VPN with impossible scheduling #374

--------------------------------By closing the meta issue in datamodel
atlanticwave-sdx/datamodel#156

[YufengXin]

closed by:
atlanticwave-sdx/datamodel#157

Will use the new tag after pce update to handle error codes

[YufengXin]

to be closed after Gretel's test.