simplewallet: reject attempts to use too low mixin early

This yields a clear error message rather then some possibly
confusing more technical errors down the line

src/wallet/wallet2.h

@@ -761,6 +761,7 @@ namespace tools
 
     uint64_t get_fee_multiplier(uint32_t priority, int fee_algorithm = -1);
     uint64_t get_per_kb_fee();
+    uint64_t adjust_mixin(uint64_t mixin);
 
     // Light wallet specific functions
     // fetch unspent outs from lw node and store in m_transfers