Handle stmtCache.Get error previously thought impossible

The statement cache is already prefilled, but it is possible for the
ctx to be canceled between when the statement is prepared and when the
statement is retrieved for use.

refs #1156

conn.go

@@ -773,8 +773,7 @@ func (c *Conn) SendBatch(ctx context.Context, b *Batch) BatchResults {
 			var err error
 			sd, err = stmtCache.Get(ctx, bi.query)
 			if err != nil {
-				// the stmtCache was prefilled from distinctUnpreparedQueries above so we are guaranteed no errors
-				panic("BUG: unexpected error from stmtCache")
+				return &batchResults{ctx: ctx, conn: c, err: err}
 			}
 		}