Because it's possible to specify the formats explicitly it doesn't make
sense not to support that. Eventually contents-based detection could be
implemented as well, but I don't want to do that just yet.
And make it return right after the first error. Also clean up some
old-style code -- the redirection should be restricted to the smallest
scope possible.
So if we convert foo.blah but explicitly tell it that it's a SPIR-V
file, it actually treats it as such, instead of complaining that `blah`
is not a known extension.
First step towards being able to operate directly on data.