Improve error handling on install

[eelcoj]

When you run the install command with an incorrect library name: rails g rails_icons:install --libraries=herocions it breaks:

 undefined method 'initializer_config' for nil (NoMethodError)

libraries.map { RailsIcons.libraries[_1.to_sym].initializer_config }.join("\n")

This is confusing as the error message is not correct.

Should improve checks on correct library names in the various generators.

Create a new error: https://github.com/Rails-Designer/rails_icons/blob/main/lib/rails_icons/errors.rb and check in each generator: https://github.com/Rails-Designer/rails_icons/tree/main/lib/generators/rails_icons