Creating a database that already exists in the glue data catalog raises an exception

moto/glue/exceptions.py

@@ -1,9 +1,15 @@
 from __future__ import unicode_literals
-from moto.core.exceptions import RESTError
+from moto.core.exceptions import JsonRESTError
 
 
-class GlueClientError(RESTError):
+class GlueClientError(JsonRESTError):
+    code = 400
 
-    def __init__(self, *args, **kwargs):
-        kwargs.setdefault('template', 'single_error')
-        super(GlueClientError, self).__init__(*args, **kwargs)
+
+class DatabaseAlreadyExistsException(GlueClientError):
+    def __init__(self):
+        self.code = 400
+        super(DatabaseAlreadyExistsException, self).__init__(
+            'DatabaseAlreadyExistsException',
+            'Database already exists.'
+        )

moto/glue/models.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 from moto.core import BaseBackend, BaseModel
 from moto.compat import OrderedDict
+from.exceptions import DatabaseAlreadyExistsException
 
 
 class GlueBackend(BaseBackend):
@@ -10,6 +11,9 @@ class GlueBackend(BaseBackend):
         self.databases = OrderedDict()
 
     def create_database(self, database_name):
+        if database_name in self.databases:
+            raise DatabaseAlreadyExistsException()
+
         database = FakeDatabase(database_name)
         self.databases[database_name] = database
         return database