DDoc: file.d: Exceptions thrown (except slurp)

Added all missing Throws:, with the exception of slurp,
which might throw pretty much anything
(at least Exception, ErrnoException, ConvException, maybe FormatException, gave up on trying to parse all the called code
(main offenders are formattedRead and the File ctor)).
Merge pull request #1268 from kiith-sa/patch-3

DDoc: file.d: Exceptions thrown (except slurp)