sqlite-utils duplicate command, closes #454, refs #449

Also made it so .duplicate() raises new NoTable exception rather than raising
an AssertionError if the source table does not exist.

sqlite_utils/db.py

@@ -1,3 +1,4 @@
+from ast import Not
 from .utils import (
     chunks,
     hash_record,
@@ -224,6 +225,11 @@ class NoObviousTable(Exception):
     pass
 
 
+class NoTable(Exception):
+    "Specified table does not exist"
+    pass
+
+
 class BadPrimaryKey(Exception):
     "Table does not have a single obvious primary key"
     pass
@@ -1482,7 +1488,8 @@ class Table(Queryable):
 
         :param new_name: Name of the new table
         """
-        assert self.exists()
+        if not self.exists():
+            raise NoTable(f"Table {self.name} does not exist")
         with self.db.conn:
             sql = "CREATE TABLE [{new_table}] AS SELECT * FROM [{table}];".format(
                 new_table=new_name,