A /stopmacro-like command would not work for this, because assuming the first line in your macro casts a spell, that spell is still "being cast" while all other lines in the macro are being processed. For example, if you have this macro:
Code:
/cast Albino Drake
/dismount
...you will always end up mounted, because /dismount (like /stopmacro) happens instantly, while /cast takes time, even for "instant cast" spells. There is no "delay" command, so there is no way to tell the macro to wait until the cast action resolves before executing the next line.