Add warning message for unknown code in fret-diagram-verbose

Add warning message for unknown code in fret-diagram-verbose

This patch does not change the printed or midi output; it only
generates a warning on the console and in the logfile.  Because
there is no change to the printed output, I felt like there was
no need for a regression test.

You can see the effect of this patch by running the following code:

\version "2.19.82"

{
 s1^\markup{
   \fret-diagram-verbose #'(
     (please harakiri 1 5)
   )
 }
}

which will create a blank fret diagram and print a warning message in the
log.

[thomasmorley651, 2019-03-16 08:34:41]

https://codereview.appspot.com/570540043/diff/548530043/scm/fret-diagrams.scm
File scm/fret-diagrams.scm (right):

https://codereview.appspot.com/570540043/diff/548530043/scm/fret-diagrams.scm...
scm/fret-diagrams.scm:267: (symbol->string my-code))))

symbol->string relies on my-code being a symbol.
Faulty user-input may be a symbol or something else, though.
Probably object->string is better?

[Carl, 2019-03-17 23:34:46]

Respond to Harm's suggestion

[Carl, 2019-03-17 23:46:46]

Testing git-cl upload