internal/worker: add String to clienterrors.Error struct

This should make the logging better when errors have another error
struct.

Fixes #3272

internal/worker/clienterrors/errors.go

@@ -1,5 +1,9 @@
 package clienterrors
 
+import (
+	"fmt"
+)
+
 const (
 	ErrorNoDynamicArgs        ClientErrorCode = 1
 	ErrorInvalidTargetConfig  ClientErrorCode = 2
@@ -46,6 +50,10 @@ type Error struct {
 	Details interface{}     `json:"details,omitempty"`
 }
 
+func (e *Error) String() string {
+	return fmt.Sprintf("Code: %d, Reason: %s, Details: %v", e.ID, e.Reason, e.Details)
+}
+
 const (
 	JobStatusSuccess        = "2xx"
 	JobStatusUserInputError = "4xx"