deleteComment and editComment do not return the correct error message when the comment does not exist

[xknown]

As reported in the ​XMLRPC list:

i noticed an odd behavior today with a few methods like deleteComment and editComment. if you give them a nonexistent comment id, they say "you're not allowed to moderate this comment," even when you are allowed. it seems like they should say the comment doesn't exist

[nacin]

In [18584]:

Check for comment existence before checking edit_comment meta cap. props xknown, fixes #18104.