Improve error message when json path selector doesn't match

go.mod

@@ -4,6 +4,6 @@ go 1.13
 
 require (
 	github.com/PaesslerAG/jsonpath v0.1.1
-	github.com/steinfletcher/apitest v1.4.6
-	github.com/stretchr/testify v1.5.1
+	github.com/steinfletcher/apitest v1.4.8
+	github.com/stretchr/testify v1.6.0
 )

go.sum

@@ -8,15 +8,13 @@ github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/steinfletcher/apitest v1.4.5 h1:BfKcS94yzsXomRV0Ojto999uziGGEz+AHh0iQWxrtu0=
-github.com/steinfletcher/apitest v1.4.5/go.mod h1:yaYc9GDlj4fa0qUUDywqAmrELlNbDrBwd36Zgo5hMZo=
-github.com/steinfletcher/apitest v1.4.6 h1:7IhnjUVTdKdNIhsHskIDNhN5JJbxuv1+e42LFDSxnjI=
-github.com/steinfletcher/apitest v1.4.6/go.mod h1:yaYc9GDlj4fa0qUUDywqAmrELlNbDrBwd36Zgo5hMZo=
+github.com/steinfletcher/apitest v1.4.8 h1:9SDVm1MCv6FRCd8xuGgoZA0TCsunN0TrXBon+ZL86wE=
+github.com/steinfletcher/apitest v1.4.8/go.mod h1:3nIZfM9GDQWGP9UGx6Zxk+LXc0DZFcZvy6+LfdhZa6U=
 github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
-github.com/stretchr/testify v1.5.1 h1:nOGnQDM7FYENwehXlg/kFVnos3rEvtKTjRvOWSzb6H4=
-github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5cxcmMvtA=
+github.com/stretchr/testify v1.6.0 h1:jlIyCplCJFULU/01vCkhKuTyc3OorI3bJFuw6obfgho=
+github.com/stretchr/testify v1.6.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
-gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

jsonpath.go

@@ -188,7 +188,7 @@ func jsonPath(reader io.Reader, expression string) (interface{}, error) {
 
 	value, err := jsonpath.Get(expression, v)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("evaluating '%s' resulted in error: '%s'", expression, err)
 	}
 	return value, nil
 }