Block parsing is now robust

Previously, parsing of a Block element was done inside its
constructor.  Parsing errors were handled via assertion checks,
but this only works in practice if there are no actual errors
in the file.

We did come across a file, however, that used EMBL-style lacing,
but the lacing was done incorrectly and so the parse asserted.
This isn't acceptable for a production system, and more a graceful
handling of parse errors was needed.

The code was restructured such that the Block object's ctor does
only trivial initialization of member variables.  A separate Parse
method was added, that is called after the object is constructed.
If the parse succeeds all is well, otherwise the object is destroyed
and the error is reported to the caller.

This commit fixes bug tracker issue #398, described here:

Change-Id: Ib95ca95d0eec08cf670b308c461e42ed8345e890
4 files changed