talksapp: remove decoration to errors at top level

The errors returned are used to decide what status code
should be set on the http response.

Fixes #441.

Change-Id: Ia01ed2186dd6d23cd38402888df7d2de3f687d0a
Reviewed-on: https://go-review.googlesource.com/28781
Reviewed-by: Chris Broadfoot <cbro@golang.org>

talksapp/main.go

diff --git a/talksapp/main.go b/talksapp/main.go
index 79e7f26..5810518 100644
--- a/talksapp/main.go
+++ b/talksapp/main.go
@@ -195,7 +195,7 @@
 	log.Infof(ctx, "Fetching presentation %s.", importPath)
 	pres, err := getPresentation(httpClient(r), importPath)
 	if err != nil {
-		return fmt.Errorf("Could not get presentation: %v", err)
+		return err
 	}
 	parser := &present.Context{
 		ReadFile: func(name string) ([]byte, error) {
@@ -207,12 +207,12 @@
 	}
 	doc, err := parser.Parse(bytes.NewReader(pres.Files[pres.Filename]), pres.Filename, 0)
 	if err != nil {
-		return fmt.Errorf("Could not parse presentation: %v", err)
+		return err
 	}
 
 	var buf bytes.Buffer
 	if err := renderPresentation(&buf, importPath, doc); err != nil {
-		return fmt.Errorf("Could not render presentation: %v", err)
+		return err
 	}
 
 	if err := memcache.Add(ctx, &memcache.Item{