Use wildcards instead of many version-specific lib names on Linux.

viewer_manifest.py's Linux_i686Manifest class has contained directives to copy
library files with names like (e.g.) "libapr-1.so.0.4.2", which means that
every update to any such library requires messing with viewer_manifest.py.
But LLManifest.path() claims to support wildcards, and it's more robust to
specify "libapr-1.so*" instead.
Unfortunately LLManifest.path()'s wildcard support only used to work for files
in the source tree (vs. the artwork tree or the build tree). The logic in
path() tries each tree in turn, relying on an exception to make it try the
next tree. This exception was raised for a nonexistent specific filename --
but it never used to raise that exception for a wildcard matching 0 files.
Instead it would simply report "0 files" and proceed, producing an invalid
viewer install.
Raise that exception for a wildcard matching nothing. This forces path() to
consider the artwork tree and the build tree, permitting us to use wildcards
in library names.
Define an exception specific to LLManifest: ManifestException rather than the
generic Python RuntimeException. Make it a subclass of RuntimeException so any
existing callers expecting to catch RuntimeException will continue to work.