Improve Messaging around Exception on Uninflectable Table Name

[SamGerber-zz]

Here's the relevant part of a stack trace from when a table has an uninflectable class_name that wasn't provided.

NoMethodError: undefined method `name' for nil:NilClass
/Users/mcgee/.gem/ruby/2.3.7/gems/live_fixtures-0.3.1/lib/live_fixtures/import.rb:80:in `initialize'
/Users/mcgee/.gem/ruby/2.3.7/gems/live_fixtures-0.3.1/lib/live_fixtures/import.rb:66:in `new'
/Users/mcgee/.gem/ruby/2.3.7/gems/live_fixtures-0.3.1/lib/live_fixtures/import.rb:66:in `block (2 levels) in import_all'
/Users/mcgee/.gem/ruby/2.3.7/gems/live_fixtures-0.3.1/lib/live_fixtures/import.rb:53:in `each'
/Users/mcgee/.gem/ruby/2.3.7/gems/live_fixtures-0.3.1/lib/live_fixtures/import.rb:53:in `block in import_all'
/Users/mcgee/.gem/ruby/2.3.7/gems/activerecord-4.2.10/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `block in transaction'
/Users/mcgee/.gem/ruby/2.3.7/gems/activerecord-4.2.10/lib/active_record/connection_adapters/abstract/transaction.rb:184:in `within_new_transaction'
/Users/mcgee/.gem/ruby/2.3.7/gems/activerecord-4.2.10/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `transaction'

It's not very helpful. Can we make it more helpful?