`Net::LDAP::DN` should raise an error more specific than `RuntimeError`

[dblessing]

Currently, Net::LDAP::DN raises RuntimeError in certain conditions. This is a very non-specific errors and makes it hard to catch LDAP exceptions without accidentally catching a more generic runtime error.

Can we add more specific error classes for these cases?

[satoryu]

Could you give us more information to reproduce your situation, the version of net-ldap and actual stack traces etc.?

[dblessing]

Everywhere in https://github.com/ruby-ldap/ruby-net-ldap/blob/master/lib/net/ldap/dn.rb where the code says raise "message" is raising a RuntimeError rather than a specific Net::LDAP error. There's also an acknowledgement in the tests that this is not the best - https://github.com/ruby-ldap/ruby-net-ldap/blob/master/test/test_dn.rb#L29