Auto merge of #6267 - christhekeele:scaffold-error-class, r=colby-swa…

…ndale

Add base error class to new gems.

Closes #6260.

Room for discussion:

- Which error class to use (`StandardError` makes sense to me)
- What formatting to use (the three-lines-with-comment seemed nicest to me)
- Whether or not using the flag to provide a different error base class is useful, and if it should validate the user's choice or not (I threw it in because it seemed harmless; is it? a boolean flag would work fine too)

---

### What was the end-user problem that led to this PR?

Libraries don't always follow best practice from discussion in linked issue.

### What was your diagnosis of the problem?

Bundler could encourage best practice by adding it to the gem scaffold.

### What is your fix for the problem, implemented in this PR?

I added a base error class to the templates, and provided a flag to change/disable this behaviour.

### Why did you choose this fix out of the possible options?

Like any best-practice-by-default, this could ruin someones workflow/go against someone's preferences so I made it as configurable as possible.

lib/bundler/templates/newgem/lib/newgem.rb.tt

@@ -6,6 +6,7 @@ require "<%= config[:namespaced_path] %>/<%= config[:underscored_name] %>"
 <%- config[:constant_array].each_with_index do |c, i| -%>
 <%= "  " * i %>module <%= c %>
 <%- end -%>
+<%= "  " * config[:constant_array].size %>class Error < StandardError; end %>
 <%= "  " * config[:constant_array].size %># Your code goes here...
 <%- (config[:constant_array].size-1).downto(0) do |i| -%>
 <%= "  " * i %>end

spec/commands/newgem_spec.rb

@@ -310,6 +310,10 @@ def create_temporary_dir(dir)
       expect(bundled_app("test_gem/lib/test_gem.rb").read).to match(%r{require "test_gem/version"})
     end
 
+    it "creates a base error class" do
+      expect(bundled_app("test_gem/lib/test_gem.rb").read).to include("class Error < StandardError")
+    end
+
     it "runs rake without problems" do
       system_gems ["rake-10.0.2"]