go.tools/cmd/vet: improve error message for build tag problems

Fixes golang/go#6979.

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/37720048

cmd/vet/buildtag.go

@@ -55,7 +55,7 @@ func checkBuildTag(name string, data []byte) {
 				continue
 			}
 			if i >= cutoff {
-				fmt.Fprintf(os.Stderr, "%s:%d: +build comment appears too late in file\n", name, i+1)
+				fmt.Fprintf(os.Stderr, "%s:%d: +build comment must appear before package clause and be followed by a blank line\n", name, i+1)
 				setExit(1)
 				continue
 			}

cmd/vet/testdata/buildtag.go

@@ -9,6 +9,6 @@
 
 package testdata
 
-// +build toolate // ERROR "build comment appears too late in file"
+// +build toolate // ERROR "build comment must appear before package clause and be followed by a blank line"
 
 var _ = 3

cmd/vet/testdata/buildtag_bad.go

@@ -7,7 +7,7 @@
 // +build !!bang // ERROR "invalid double negative in build constraint"
 // +build @#$ // ERROR "invalid non-alphanumeric build constraint"
 
-// +build toolate // ERROR "build comment appears too late in file"
+// +build toolate // ERROR "build comment must appear before package clause and be followed by a blank line"
 package bad
 
 // This is package 'bad' rather than 'main' so the erroneous build