Raise a custom Rackstash::NotImplementedHereError instead of NotImplementedError

Using the core `NotInmplementedError` is not desireable since its
documentation includes:

> Note that if `fork` raises a `NotImplementedError`, then
> `respond_to?(:fork)` returns `false`.

Since we are responding to the method but still raise an error, our
usage of the exception does not fulfill its documentation.

A custom error instead of a default `NoMethodError` is still desireable
since it significantly helps with debugging. With a different Exception,
we make it clear that a method is expected to be there and just wasn't
implemented by a subclass as opposed to the caller just using an object
wrong and calling entirely unexpected methods on it.

lib/rackstash.rb

@@ -10,6 +10,14 @@ require 'set'
 require 'rackstash/version'
 
 module Rackstash
+  # A custom error which is raised by methods which need to be implemented
+  # elsewhere, usually in a subclass. Please refer to the documentation of the
+  # method which raised this error for details.
+  #
+  # Note that this error is not a `StandardError` and will not be rescued
+  # unless it or any of its ancestors, e.g. `Exception` is specified explicitly.
+  class NotImplementedHereError < ScriptError; end
+
   SEVERITIES = [
     DEBUG = 0,
     INFO = 1,

lib/rackstash/adapters/adapter.rb

@@ -82,15 +82,16 @@ module Rackstash
 
       # Write a single log line to the log device.
       #
-      # This method needs to be overwritten in child classes to write the
+      # This method needs to be overwritten in adapter sub classes to write the
-      # encoded log event to the adapter's device. By default, this method
+      # encoded log event to the adapter's device. When not overwritten, this
-      # raises a `NotImplementedError`.
+      # method raises a {NotImplementedHereError}.
       #
       # @param log [Object] the encoded log event
       # @return [void]
-      # @raise NotImplementedError
+      # @raise NotImplementedHereError
       def write_single(log)
-        raise NotImplementedError, 'write needs to be implemented in a sub class'
+        raise NotImplementedHereError, 'write_single needs to be implemented ' +
+          'in the actual adapter subclass'
       end
 
       private

spec/rackstash/adapters/adapter_spec.rb

@@ -47,7 +47,8 @@ describe Rackstash::Adapters::Adapter do
 
   describe '#write_single' do
     it 'is not implemented in the abstract base class' do
-      expect { adapter.write('something') }.to raise_error(NotImplementedError)
+      expect { adapter.write('something') }
+        .to raise_error(Rackstash::NotImplementedHereError)
     end
   end
 end

spec/rackstash_spec.rb

@@ -8,6 +8,12 @@
 require 'spec_helper'
 
 describe Rackstash do
+  describe Rackstash::NotImplementedHereError do
+    it 'inherits from ScriptError' do
+      expect(described_class.superclass).to equal ScriptError
+    end
+  end
+
   it 'defines PROGRAME with the correct version' do
     expect(Rackstash::PROGNAME).to match %r{\Arackstash/v\d+(\..+)*\z}
     expect(Rackstash::PROGNAME).to be_frozen