oracle: report a better error if query is outside GOROOT/GOPATH

Change-Id: I2f724b5015021be8bf1fccb518f17cc011d687e8
Reviewed-on: https://go-review.googlesource.com/9546
Reviewed-by: David Chase <drchase@google.com>

oracle/what.go

@@ -152,7 +152,8 @@ func guessImportPath(filename string, buildContext *build.Context) (srcdir, impo
 		}
 	}
 	if srcdir == "" {
-		err = fmt.Errorf("can't find package for file %s", filename)
+		err = fmt.Errorf("directory %s is not beneath GOROOT or GOPATH: %s",
+			filepath.Dir(absFile), strings.Join(buildContext.SrcDirs(), ", "))
 	}
 	return
 }