Print exception cause when inspecting with backtrace

spec/std/exception_spec.cr

@@ -18,4 +18,20 @@ describe "Exception" do
     ex = FooError.new("foo?")
     ex.inspect.should eq("#<FooError:foo? -- bar!>")
   end
+
+  it "inspects with cause" do
+    cause = Exception.new("inner")
+    ex = expect_raises(Exception, "wrapper") do
+      begin
+        raise cause
+      rescue ex
+        raise Exception.new("wrapper", cause: ex)
+      end
+    end
+
+    ex.cause.should be(cause)
+    ex.inspect_with_backtrace.should contain("wrapper")
+    ex.inspect_with_backtrace.should contain("Caused by")
+    ex.inspect_with_backtrace.should contain("inner")
+  end
 end

src/exception.cr

@@ -59,6 +59,12 @@ class Exception
       io.print "  from "
       io.puts frame
     end
+
+    if cause = @cause
+      io << "Caused by: "
+      cause.inspect_with_backtrace(io)
+    end
+
     io.flush
   end
 end