#4758 No warning upon failed ‘align[Above|Below]Context’

[Simon Albrecht]

In this example

\version "2.19.35"
\new StaffGroup <<
  \new Staff = "A" { a'1 a' }
  { \skip 1 \new Staff = "C" \with { alignAboveContext = "B" } { c''1 } }
  { \skip 1 \new Staff = "B" \with { alignAboveContext = "A" } { b'1 } }
>>

Lily can’t find context "B" when creating "C", so she silently prints "C" below "A". There should be a warning about this.