[5418] Improved error handling of invalid subnet prefix lengths

[5418] Improved error handling of invalid subnet prefix lengths

src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    SubnetConfigParser::createSubnet() - explicitly catch
    exceptions thrown by lexical_cast and added logic to
    catch values > 256

src/bin/dhcp4/tests/config_parser_unittest.cc
src/bin/dhcp6/tests/config_parser_unittest.cc
    badSubnetValues() - new test that checks
    several invalid subnet value scenarios