Fixed some error states and error messages, read more

By leaving 'null' on the stack, it won't cause stack underflows in a
bunch of erroneous situations. This will allow the repl (and other
situations) to continue if they want to.

I've also fixed some error messages in toy_table.c, which were formatted
badly.

Closes #162

source/toy_opcodes.h

@@ -45,7 +45,6 @@ typedef enum Toy_OpcodeType {
 	TOY_OPCODE_PRINT,
 	TOY_OPCODE_CONCAT,
 	TOY_OPCODE_INDEX,
-	//URGENT: clear the program stack - much needed
 
 	//meta instructions
 	TOY_OPCODE_PASS,