Ambiguous (de)crescendo in MIDI error

[Knute Snortum]

Something that is seen a lot in 19th and 20th century music is a crescendo
or decrescendo without going to a specific dynamic.  The performer is given
the freedom to decide how much to change the dynamics.  This causes an
error message: "programming error: Impossible or ambiguous (de)crescendo in
MIDI."

%%% BEGIN
\version "2.19.10"

\score {
  \relative c' {
    c1 \p | c4 \< d e f | g \> f e d | c1 \p
  }
  \layout {}
  \midi {}
}
%%% END

This error message should be a warning and give some line number to the
offending code.

Knute Snortum
(via Gmail)