[Gutter] Don't allow the 'line-number' gutter to be destroyed

spec/gutter-spec.coffee

@@ -62,3 +62,7 @@ describe 'Gutter', ->
         didDestroy = true
       gutter.destroy()
       expect(didDestroy).toBe true
+
+    it 'does not allow destroying the line-number gutter', ->
+      gutter = new Gutter mockGutterContainer, {name: 'line-number'}
+      expect(gutter.destroy).toThrow()

src/gutter.coffee

@@ -23,9 +23,12 @@ class Gutter
     @emitter = new Emitter
 
   destroy: ->
-    @gutterContainer.removeGutter(this)
-    @emitter.emit 'did-destroy'
-    @emitter.dispose()
+    if @name is 'line-number'
+      throw new Error('The line-number gutter cannot be destroyed.')
+    else
+      @gutterContainer.removeGutter(this)
+      @emitter.emit 'did-destroy'
+      @emitter.dispose()
 
   hide: ->
     if @visible