FeaMerge errors could be more useful

Came up during review, but I'm going to defer for now; the short version is that it would be nice if our merge errors could name masters or provide paths to the relevant files, instead of using fairly meaningless indices as identifiers.

the master index in MergeError is the position after this swap, so when the merge fails the user gets "master 3: ..." and no way to tell which features.fea that is. fontbe knows the path and location of each master at this point; can we map the index back to the source, or attach them here, before this reaches crater?

_Originally posted by @anthrotype in https://github.com/googlefonts/fontc/pull/2125#discussion_r3991861798_

Start with MergeError and trace the master-index swap in fontbe before the error reaches crater. Check how fontbe knows each master’s source path and location, then determine how merge failures can expose that information instead of post-swap indices. Done means the reported master identifies its relevant features.fea source or path.

Written by the indexing model from the issue text.