ENH: support low-level reading of raw binary blocks (#1378)

- symmetric with writeRaw() etc for the output stream. These are
  methods that are not required by normal users, but which provide
  a finer granularity for reading.

- additional information about the current stream position when
  endList fails. This may help when tracing errors.

STYLE: change return type of beginRaw()/endRaw() to bool

- was of type Ostream& for output, but return value was unused.
  Having bool may be better for encapsulating logic

STYLE: remove unused Istream::readEndBegin() function