validate list of commands when using ;

[dcbaker]

When passing commands via the : prompt, the commands are unvalidated. If the first X commands are valid then the execution will stop when attempting to instantiate the invalid command.

As disscussed in #966 it is important to not instantiate the commands to validate them.

[pazz]

for some context: We would like that a commandline say bclose; dontexist; taglist doesn't even start with the first command since the second one is syntactic garbage.
One caveat is that a command may be OK in one mode, but not in another.
So that for instance taglist;compose;send;send should really not be OK, as the second send would be instantiated out in a taglist buffer, where it makes no sense.