internal/fetch: add missing derrors.Wrap

Change-Id: I92f532ea84912c7c56933ae93311aa69b433bbcb
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/256307
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/fetch/load.go b/internal/fetch/load.go
index 5636541..af19039 100644
--- a/internal/fetch/load.go
+++ b/internal/fetch/load.go
@@ -64,7 +64,8 @@
 //
 // If the package is fine except that its documentation is too large, loadPackage
 // returns both a package and a non-nil error with dochtml.ErrTooLarge in its chain.
-func loadPackage(ctx context.Context, zipGoFiles []*zip.File, innerPath string, sourceInfo *source.Info, modInfo *dochtml.ModuleInfo) (*goPackage, error) {
+func loadPackage(ctx context.Context, zipGoFiles []*zip.File, innerPath string, sourceInfo *source.Info, modInfo *dochtml.ModuleInfo) (_ *goPackage, err error) {
+	defer derrors.Wrap(&err, "loadPackage(ctx, zipGoFiles, %q, sourceInfo, modInfo)", innerPath)
 	ctx, span := trace.StartSpan(ctx, "fetch.loadPackage")
 	defer span.End()
 	for _, env := range goEnvs {
diff --git a/internal/fetch/package.go b/internal/fetch/package.go
index f4479fa..d803a9b 100644
--- a/internal/fetch/package.go
+++ b/internal/fetch/package.go
@@ -56,6 +56,7 @@
 // * the particular set of build contexts we consider (goEnvs)
 // * whether the import path is valid.
 func extractPackagesFromZip(ctx context.Context, modulePath, resolvedVersion string, r *zip.Reader, d *licenses.Detector, sourceInfo *source.Info) (_ []*goPackage, _ []*internal.PackageVersionState, err error) {
+	defer derrors.Wrap(&err, "extractPackagesFromZip(ctx, %q, %q, r, d)", modulePath, resolvedVersion)
 	ctx, span := trace.StartSpan(ctx, "fetch.extractPackagesFromZip")
 	defer span.End()
 	defer func() {
diff --git a/internal/fetch/readme.go b/internal/fetch/readme.go
index 9b07828..edcf316 100644
--- a/internal/fetch/readme.go
+++ b/internal/fetch/readme.go
@@ -12,11 +12,13 @@
 	"strings"
 
 	"golang.org/x/pkgsite/internal"
+	"golang.org/x/pkgsite/internal/derrors"
 )
 
 // extractReadmesFromZip returns the file path and contents of all files from r
 // that are README files.
-func extractReadmesFromZip(modulePath, resolvedVersion string, r *zip.Reader) ([]*internal.Readme, error) {
+func extractReadmesFromZip(modulePath, resolvedVersion string, r *zip.Reader) (_ []*internal.Readme, err error) {
+	defer derrors.Wrap(&err, "extractReadmesFromZip(ctx, %q, %q, r)", modulePath, resolvedVersion)
 	var readmes []*internal.Readme
 	for _, zipFile := range r.File {
 		if isReadme(zipFile.Name) {