API: added error class

It can be used when lookup fails.

## Usage

```ruby
scope_class = Scope.for(object.class) or
  raise Magic::Lookup::Error.for(object, Scope)
```

lib/magic/lookup.rb

@@ -6,6 +6,8 @@
 
 module Magic
 	module Lookup
+		autoload :Error, 'magic/lookup/error'
+
 		include Memery
 
 		memoize def for object_class

lib/magic/lookup/error.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Magic
+	module Lookup
+		class Error < NameError
+			def self.for object, lookup_class
+				default_name = lookup_class.name_for object.class
+
+				new "no #{lookup_class} found for #{object.class}, default name is #{default_name}",
+						default_name, receiver: object
+			end
+		end
+	end
+end

sig/magic/lookup/error.rbs

@@ -0,0 +1,7 @@
+module Magic
+	module Lookup
+		class Error < NameError
+			def self.for: (any) -> Error
+		end
+	end
+end

spec/magic/lookup/error_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class Scope
+	extend Magic::Lookup
+
+	def self.name_for(object_class) = "#{object_class}Scope"
+end
+
+module Magic
+	module Lookup
+		RSpec.describe Error do
+			subject { described_class.new }
+
+			describe '.for', :method do
+				its([[], Scope]) do
+					is_expected.to be_instance_of described_class
+				end
+
+				its([[], Scope]) do
+					is_expected.to be_a StandardError
+				end
+
+				its([[], Scope]) do
+					is_expected.to have_attributes to_s: /no Scope found/,
+							name: 'ArrayScope', receiver: []
+				end
+			end
+		end
+	end
+end