Clarify output when printing nested exception traces

Since v10.2.0, if an exception has a nested cause exception, the cause is also
displayed in the trace output.[1]

For heavily-nested exceptions, this output can be quite lengthy - for example,
Rails migrations nest DB errors twice over, resulting in an error message and
backtrace repeated three times.

To break up this output and make it clearer what each individual backtrace
relates to, this adds whitespace and a "Caused by:" label to each nested
exception being displayed.

To prevent "Caused by:" labels occurring on their own, I've moved the
exception loop shortcut return into the `#display_cause_details` method. This
doesn't alter the behaviour of the shortcut, as only the first exception will
be unconditionally printed (which was already the case, as the first exception
can't be already seen).

[1] fbb22e7
[2] 57c932c

lib/rake/application.rb

@@ -207,13 +207,22 @@ def display_error_message(ex) # :nodoc:
     end
 
     def display_exception_details(ex) # :nodoc:
-      seen = Thread.current[:rake_display_exception_details_seen] ||= []
-      return if seen.include? ex
-      seen << ex
+      display_exception_details_seen << ex
 
       display_exception_message_details(ex)
       display_exception_backtrace(ex)
-      display_exception_details(ex.cause) if has_cause?(ex)
+      display_cause_details(ex.cause) if has_cause?(ex)
+    end
+
+    def display_cause_details(ex) # :nodoc:
+      return if display_exception_details_seen.include? ex
+
+      trace "\nCaused by:"
+      display_exception_details(ex)
+    end
+
+    def display_exception_details_seen
+      Thread.current[:rake_display_exception_details_seen] ||= []
     end
 
     def has_cause?(ex) # :nodoc:

test/test_rake_application.rb

@@ -97,6 +97,7 @@ def test_display_exception_details_cause
 
     assert_empty out
 
+    assert_match "Caused by:", err
     assert_match "cause a", err
     assert_match "cause b", err
   end