internal/vulncheck: improve error message for fetching vulns

Fixes golang/go#64681

Change-Id: Ie65279bd557d9477b62e7aedcf6ec620536e1e18
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/549375
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Maceo Thompson <maceothompson@google.com>

Author: zpavlinovic

all_test.go

@@ -96,11 +96,9 @@ func TestGovulncheck(t *testing.T) {
 	case nil:
 		t.Log("govulncheck: no vulnerabilities detected")
 	case interface{ ExitCode() int }:
-		if err.ExitCode() != 0 {
-			t.Errorf("govulncheck ./... failed with exit code: %d", err.ExitCode())
-		}
+		t.Errorf("govulncheck: unexpected exit code %d and error %v", err.ExitCode(), err)
 	default:
-		t.Errorf("unrecognized govulncheck command error: %v", err)
+		t.Errorf("govulncheck: abruptly failed with error %v", err)
 	}
 }
 

internal/vulncheck/fetch.go

@@ -6,6 +6,7 @@ package vulncheck
 
 import (
 	"context"
+	"fmt"
 
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/vuln/internal/client"
@@ -25,7 +26,7 @@ func FetchVulnerabilities(ctx context.Context, c *client.Client, modules []*pack
 	}
 	resps, err := c.ByModules(ctx, mreqs)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("fetching vulnerabilities: %v", err)
 	}
 	var mv []*ModVulns
 	for i, resp := range resps {