Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

credentials/error.go

@@ -0,0 +1,37 @@
+package credentials
+
+// ErrCredentialsNotFound standarizes the not found error, so every helper returns
+// the same message and docker can handle it properly.
+const errCredentialsNotFoundMessage = "credentials not found in native keychain"
+
+// errCredentialsNotFound represents an error
+// raised when credentials are not in the store.
+type errCredentialsNotFound struct{}
+
+// Error returns the standard error message
+// for when the credentials are not in the store.
+func (errCredentialsNotFound) Error() string {
+	return errCredentialsNotFoundMessage
+}
+
+// NewErrCredentialsNotFound creates a new error
+// for when the credentials are not in the store.
+func NewErrCredentialsNotFound() error {
+	return errCredentialsNotFound{}
+}
+
+// IsErrCredentialsNotFound returns true if the error
+// was caused by not having a set of credentials in a store.
+func IsErrCredentialsNotFound(err error) bool {
+	_, ok := err.(errCredentialsNotFound)
+	return ok
+}
+
+// IsErrCredentialsNotFoundMessage returns true if the error
+// was caused by not having a set of credentials in a store.
+//
+// This function helps to check messages returned by an
+// external program via its standard output.
+func IsErrCredentialsNotFoundMessage(err string) bool {
+	return err == errCredentialsNotFoundMessage
+}