LibJS: Keep current exception in a call frame register

Instead of keeping it in a Bytecode::Interpreter member, move it into
a dedicated call frame register.

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -252,7 +252,7 @@ private:
     NonnullOwnPtr<IdentifierTable> m_identifier_table;
     NonnullOwnPtr<RegexTable> m_regex_table;
 
-    u32 m_next_register { 2 };
+    u32 m_next_register { 3 };
     u32 m_next_block { 1 };
     u32 m_next_property_lookup_cache { 0 };
     u32 m_next_global_variable_cache { 0 };