Fix crash when a stencil routine is missing

Fix crash when a stencil routine is missing

example: using embedded-ps with -dbackend=svg

- Remove obsolete code from lily.scm that was
used before the output backends took their current
form.

- Remove exported bindings from output-ps.scm; the
output backends no longer use a `public interface',
since all work is done with ly:outputter-dump-stencil,
which uses the modules directly.

- Reorganize define-stencil-commands.scm and add a couple
of missing stencil commands.

- Use a generic procedure (backend-testing) for the
current output backend to emit warnings if a stencil
routine is missing.  Additionally, return an empty
string (null string) for missing routines.

- Create a new program option (-dwarning-as-error) that
will cause LilyPond to exit with an error for all warnings and programming_errors.  It is disabled by default.

- In the `placebox' stencil routine, check for an empty
string before execution.

[hanwenn, 2009-07-02 02:36:41]

Can this be made so that the regtests and similar crash when an unknown stencil
expression is executed?  Perhaps a -d option could help here.

http://codereview.appspot.com/83046/diff/1/5
File scm/output-svg.scm (right):

http://codereview.appspot.com/83046/diff/1/5#newcode284
Line 284: (if (< 0 (string-length expr))
isnt there a function string-empty? or string-null? (or similar) to do this in a
more roundabout name?

[joeneeman, 2009-07-02 05:19:36]

http://codereview.appspot.com/83046/diff/1/3
File scm/output-ps.scm (right):

http://codereview.appspot.com/83046/diff/1/3#newcode58
Line 58: (ly:all-output-backend-commands)))))
Perhaps this could be a macro (so that you don't need to c&p for every backend).
And if you add a -d option like Han-Wen suggested then you can get rid of the
(if #t ...)

[joeneeman, 2009-07-16 05:42:25]

lgtm