cmd/go/internal/get: avoid panic in metaImportsForPrefix if web.Get fails

Fixes #34049

Change-Id: I817b83ee2d0ca6d01ec64998f14bc4f32e365d66
Reviewed-on: https://go-review.googlesource.com/c/go/+/193259
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/go/internal/get/vcs.go

@@ -904,7 +904,7 @@ func metaImportsForPrefix(importPrefix string, mod ModuleMode, security web.Secu
 		}
 		resp, err := web.Get(security, url)
 		if err != nil {
-			return setCache(fetchResult{url: url, err: fmt.Errorf("fetch %s: %v", resp.URL, err)})
+			return setCache(fetchResult{url: url, err: fmt.Errorf("fetching %s: %v", importPrefix, err)})
 		}
 		body := resp.Body
 		defer body.Close()
@@ -913,7 +913,7 @@ func metaImportsForPrefix(importPrefix string, mod ModuleMode, security web.Secu
 			return setCache(fetchResult{url: url, err: fmt.Errorf("parsing %s: %v", resp.URL, err)})
 		}
 		if len(imports) == 0 {
-			err = fmt.Errorf("fetch %s: no go-import meta tag", url)
+			err = fmt.Errorf("fetching %s: no go-import meta tag found in %s", importPrefix, resp.URL)
 		}
 		return setCache(fetchResult{url: url, imports: imports, err: err})
 	})

src/cmd/go/testdata/script/get_insecure_redirect.txt

 # golang.org/issue/29591: 'go get' was following plain-HTTP redirects even without -insecure.
+# golang.org/issue/34049: 'go get' would panic in case of an insecure redirect in GOPATH mode
 
 [!net] skip
 [!exec:git] skip
 
-env GO111MODULE=on
-env GOPROXY=direct
-env GOSUMDB=off
+env GO111MODULE=off
 
 ! go get -d vcs-test.golang.org/insecure/go/insecure
 stderr 'redirected .* to insecure URL'

src/cmd/go/testdata/script/mod_get_insecure_redirect.txt

+# golang.org/issue/29591: 'go get' was following plain-HTTP redirects even without -insecure.
+
+[!net] skip
+[!exec:git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+! go get -d vcs-test.golang.org/insecure/go/insecure
+stderr 'redirected .* to insecure URL'
+
+go get -d -insecure vcs-test.golang.org/insecure/go/insecure