Eliminate ManifestError for wildcards matching 0 files.

Turns out that some (many?) wildcard LLManifest.path(wildcard) calls are "just
in case": sweep up any (e.g.) "*.tga" files there may be, but no problem if
there are none.
Change path() logic so it tries the next tree (source, artwork, build) if
either a specific (non-wildcard) filename doesn't exist, as now, OR if a
wildcard matches 0 files in the current tree. This continues to support "just
in case" wildcards, while permitting wildcards to work in the artwork and
build trees as well as the source tree.
Use a more specific exception than ManifestError for missing file. Only in
that case should we try the next tree. Any other ManifestError should
propagate.