Simplify serialize.h's exception handling

Remove the 'state' and 'exceptmask' from serialize.h's stream implementations,
as well as related methods.

As exceptmask always included 'failbit', and setstate was always called with
bits = failbit, all it did was immediately raise an exception. Get rid of
those variables, and replace the setstate with direct exception throwing
(which also removes some dead code).

As a result, good() is never reached after a failure (there are only 2
calls, one of which is in tests), and can just be replaced by !eof().

fail(), clear(n) and exceptions() are just never called. Delete them.

src/main.cpp

@@ -3228,7 +3228,7 @@ bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp)
             }
         }
         uint64_t nRewind = blkdat.GetPos();
-        while (blkdat.good() && !blkdat.eof()) {
+        while (!blkdat.eof()) {
             boost::this_thread::interruption_point();
 
             blkdat.SetPos(nRewind);