Make fatalerror and yyerror consistent

There are two ways in which the assembly process can fail:

1. If there is a really big problem that compromises the whole process,
   the assembler has to stop right there and generate an error message.
   This happens with unterminated REPT loops, macros, etc.

2. If the problem isn't that big and the process can still continue,
   even though the final result is invalid, the assembler can try to
   continue and warn the user about all errors it finds in the code.

This patch clarifies the use of each function and replaces the function
used in two places by the correct one.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/asm/lexer.c

@@ -535,7 +535,7 @@ yylex_ReadBracketedSymbol(char *dest, size_t index)
 	if (*pLexBuffer == '}')
 		pLexBuffer++;
 	else
-		yyerror("Missing }");
+		fatalerror("Missing }");
 
 	return length;
 }
@@ -601,7 +601,7 @@ yylex_ReadQuotedString()
 	if (*pLexBuffer == '"')
 		pLexBuffer++;
 	else
-		yyerror("Unterminated string");
+		fatalerror("Unterminated string");
 }
 
 ULONG