Print a more helpful message if the daemon crashes

Instead of

   error: unexpected end-of-file

you now get

   error: Nix daemon disconnected unexpectedly (maybe it crashed?)

src/libstore/remote-store.cc

@@ -67,6 +67,7 @@ void RemoteStore::initConnection(Connection & conn)
 {
     /* Send the magic greeting, check for the reply. */
     try {
+        conn.from.endOfFileError = "Nix daemon disconnected unexpectedly (maybe it crashed?)";
         conn.to << WORKER_MAGIC_1;
         conn.to.flush();
         StringSink saved;

src/libutil/serialise.cc

@@ -132,7 +132,7 @@ size_t FdSource::readUnbuffered(char * data, size_t len)
         n = ::read(fd, data, len);
     } while (n == -1 && errno == EINTR);
     if (n == -1) { _good = false; throw SysError("reading from file"); }
-    if (n == 0) { _good = false; throw EndOfFile("unexpected end-of-file"); }
+    if (n == 0) { _good = false; throw EndOfFile(endOfFileError); }
     read += n;
     return n;
 }

src/libutil/serialise.hh

@@ -153,12 +153,13 @@ struct FdSource : BufferedSource
 {
     int fd;
     size_t read = 0;
+    std::string endOfFileError{"unexpected end-of-file"};
 
     FdSource() : fd(-1) { }
     FdSource(int fd) : fd(fd) { }
-    FdSource(FdSource&&) = default;
+    FdSource(FdSource &&) = default;
 
-    FdSource& operator=(FdSource && s)
+    FdSource & operator=(FdSource && s)
     {
         fd = s.fd;
         s.fd = -1;