stbt match: Refactor: Remove obsolete exception-handling
stbt match: Refactor: Remove obsolete exception-handling

I assume this was left over from early days when we used
`stbt.wait_for_match` instead of `stbt.match`.

`stbt.match_all` (and `stbt.match`) never raise `stbt.UITestError`.
If the reference image doesn't exist they'll raise IOError.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester