fix(errors): spurious Bluebird warnings (#8408)

The way Sequelize instantiates some of its Error subclasses
confuses Bluebird into thinking that Promises are rejected
with non-Error objects.

See https://github.com/petkaantonov/bluebird/issues/990

lib/errors/index.js

@@ -11,7 +11,6 @@ class BaseError extends Error {
   constructor(message) {
     super(message);
     this.name = 'SequelizeBaseError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }
@@ -86,7 +85,6 @@ class OptimisticLockError extends BaseError {
     options.message = options.message || 'Attempting to update a stale model instance: ' + options.modelName;
     super(options);
     this.name = 'SequelizeOptimisticLockError';
-    this.message = options.message;
     /**
      * The name of the model on which the update was attempted
      * @type {string}
@@ -156,7 +154,6 @@ class UniqueConstraintError extends ValidationError {
     super(options.message, options.errors);
 
     this.name = 'SequelizeUniqueConstraintError';
-    this.message = options.message;
     this.errors = options.errors;
     this.fields = options.fields;
     this.parent = options.parent;
@@ -200,7 +197,7 @@ class ExclusionConstraintError extends DatabaseError {
     super(options.parent);
     this.name = 'SequelizeExclusionConstraintError';
 
-    this.message = options.message || options.parent.message;
+    this.message = options.message || options.parent.message || '';
     this.constraint = options.constraint;
     this.fields = options.fields;
     this.table = options.table;
@@ -340,7 +337,6 @@ class InstanceError extends BaseError {
   constructor(message) {
     super(message);
     this.name = 'SequelizeInstanceError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }
@@ -353,7 +349,6 @@ class EmptyResultError extends BaseError {
   constructor(message) {
     super(message);
     this.name = 'SequelizeEmptyResultError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }
@@ -366,7 +361,6 @@ class EagerLoadingError extends BaseError {
   constructor(message) {
     super(message);
     this.name = 'SequelizeEagerLoadingError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }
@@ -379,7 +373,6 @@ class AssociationError extends BaseError {
   constructor(message) {
     super(message);
     this.name = 'SequelizeAssociationError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }
@@ -391,7 +384,6 @@ class QueryError extends BaseError {
   constructor(message) {
     super(message);
     this.name = 'SequelizeQueryError';
-    this.message = message;
     Error.captureStackTrace(this, this.constructor);
   }
 }