testing: Make handlers.Error obey the standard 'error' interface

And return these types from test helper functions

internal/handlers/common_test.go

@@ -3,7 +3,6 @@ package handlers_test
 import (
 	"database/sql"
 	"encoding/json"
-	"fmt"
 	"github.com/aclindsa/moneygo/internal/config"
 	"github.com/aclindsa/moneygo/internal/db"
 	"github.com/aclindsa/moneygo/internal/handlers"
@@ -63,7 +62,7 @@ func create(client *http.Client, input, output TransactType, urlsuffix, key stri
 		return err
 	}
 	if e.ErrorId != 0 || len(e.ErrorString) != 0 {
-		return fmt.Errorf("Error when creating %s: %+v", urlsuffix, e)
+		return &e
 	}
 
 	err = output.Read(string(body))
@@ -92,7 +91,7 @@ func read(client *http.Client, output TransactType, urlsuffix, key string) error
 		return err
 	}
 	if e.ErrorId != 0 || len(e.ErrorString) != 0 {
-		return fmt.Errorf("Error when updating %s: %+v", urlsuffix, e)
+		return &e
 	}
 
 	err = output.Read(string(body))
@@ -125,7 +124,7 @@ func update(client *http.Client, input, output TransactType, urlsuffix, key stri
 		return err
 	}
 	if e.ErrorId != 0 || len(e.ErrorString) != 0 {
-		return fmt.Errorf("Error when updating %s: %+v", urlsuffix, e)
+		return &e
 	}
 
 	err = output.Read(string(body))
@@ -154,7 +153,7 @@ func remove(client *http.Client, urlsuffix, key string) error {
 		return err
 	}
 	if e.ErrorId != 0 || len(e.ErrorString) != 0 {
-		return fmt.Errorf("Error when removing %s: %+v", urlsuffix, e)
+		return &e
 	}
 
 	return nil

internal/handlers/errors.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"encoding/json"
+	"fmt"
 	"log"
 	"net/http"
 	"strings"
@@ -12,6 +13,10 @@ type Error struct {
 	ErrorString string
 }
 
+func (e *Error) Error() string {
+	return fmt.Sprintf("Error %d: %s", e.ErrorId, e.ErrorString)
+}
+
 func (e *Error) Read(json_str string) error {
 	dec := json.NewDecoder(strings.NewReader(json_str))
 	return dec.Decode(e)