invalid db error and value and invalid value length error (#4151)

association.go

@@ -1,7 +1,6 @@
 package gorm
 
 import (
-	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -441,7 +440,7 @@ func (association *Association) saveAssociation(clear bool, values ...interface{
 				break
 			}
 
-			association.Error = errors.New("invalid association values, length doesn't match")
+			association.Error = ErrInvalidValueOfLength
 			return
 		}
 

callbacks.go

@@ -104,7 +104,7 @@ func (p *processor) Execute(db *DB) {
 			stmt.ReflectValue = stmt.ReflectValue.Elem()
 		}
 		if !stmt.ReflectValue.IsValid() {
-			db.AddError(fmt.Errorf("invalid value"))
+			db.AddError(ErrInvalidValue)
 		}
 	}
 

errors.go

@@ -31,4 +31,10 @@ var (
 	ErrEmptySlice = errors.New("empty slice found")
 	// ErrDryRunModeUnsupported dry run mode unsupported
 	ErrDryRunModeUnsupported = errors.New("dry run mode unsupported")
+	// ErrInvaildDB invalid db
+	ErrInvaildDB = errors.New("invalid db")
+	// ErrInvalidValue invalid value
+	ErrInvalidValue = errors.New("invalid value")
+	// ErrInvalidValueOfLength invalid values do not match length
+	ErrInvalidValueOfLength = errors.New("invalid association values, length doesn't match")
 )

gorm.go

@@ -3,7 +3,6 @@ package gorm
 import (
 	"context"
 	"database/sql"
-	"errors"
 	"fmt"
 	"sync"
 	"time"
@@ -331,7 +330,7 @@ func (db *DB) DB() (*sql.DB, error) {
 		return sqldb, nil
 	}
 
-	return nil, errors.New("invalid db")
+	return nil, ErrInvaildDB
 }
 
 func (db *DB) getInstance() *DB {