Clear fret-diagram- and harp-pedal-input-strings from whitespace

[Thomas Morley]

The following

\version "2.19.25"

\markup \fret-diagram
    #"s:2;g:5;
6-3;5-5;4-5;3-4;2-3;1-3;"

returns:
GNU LilyPond 2.19.25
Processing `test.ly'
Parsing.../home/harm/lilypond-git/build/out/share/lilypond/current/scm/fret-diagrams.scm:963:38:
In procedure cadr in expression (cadr this-list):
/home/harm/lilypond-git/build/out/share/lilypond/current/scm/fret-diagrams.scm:963:38:
Wrong type (expecting pair): ()

As an enhancement we should clear the input-string from
whitespace-characters before further processing it.
More helpful messages for typos would be helpful as well.

Partly same for harp-pedal.

cc-ing James


Cheers,
  Harm