Increase test coverage, mock libcurl & uniformize error messages

curl.go

@@ -0,0 +1,13 @@
+package cmrdr
+
+import (
+	curl "github.com/andelf/go-curl"
+)
+
+// Curler is an interface that implements the CURL interface of the go-curl library
+// Used for mocking
+type Curler interface {
+	Setopt(opt int, param interface{}) error
+	Perform() error
+	Getinfo(info curl.CurlInfo) (interface{}, error)
+}