Fix ses-delete-blanks to delete only blanks + documentation.

Fix ses-delete-blanks to delete only blanks + documentation.

* doc/misc/ses.texi (Quick Tutorial): Mention the '!'
'ses-range' modifier as an alternative to 'ses+'.
(Advanced Features): Add a refernce to node 'Nonrelocatable
references' concerning function 'ses-rename-cell'.
(Standard formula functions): Mention the '!' 'ses-range'
modifier as an alternative to 'ses-delete-blanks'.
(More on cell printing): Fix fallback printer
definition.  Minor editorial formatting changes.
(Nonrelocatable references): Document the use of
'ses-rename-cell' as a better way to make cell reference
non-relocatable.
(The data area): Document the presence of local printer
definitions in the data area.

* lisp/ses.el (ses-delete-blanks): Do not remove
*error*.  Any error in an argument should propagate into the
using formula rather than being silently hidden !