Dap: prevent crash by checking if library was not found (bgo#614039)
Dap: prevent crash by checking if library was not found (bgo#614039)

As the stacktrace in comment#1 of bug#614039 shows
(https://bugzilla.gnome.org/show_bug.cgi?id=614039#c1)
it could happen that the source is not found by the
RemoveLibrary() method.

Note: this had a slightly confusing exception message
because in old versions of mono there was a bug about
this, which was fixed in mono, actually by yours truly:
https://github.com/mono/mono/commit/caf7a8b2c0b0288b19f90b1e43092d4da4083d22

This fixes the first part of bgo#614039.

ixfalia/banshee,GNOME/banshee,GNOME/banshee,ixfalia/banshee,Carbenium/banshee,arfbtwn/banshee,Carbenium/banshee,Carbenium/banshee,Carbenium/banshee,Carbenium/banshee,GNOME/banshee,ixfalia/banshee,arfbtwn/banshee,GNOME/banshee,Carbenium/banshee,arfbtwn/banshee,arfbtwn/banshee,GNOME/banshee,arfbtwn/banshee,arfbtwn/banshee,ixfalia/banshee,ixfalia/banshee,ixfalia/banshee,GNOME/banshee,GNOME/banshee,ixfalia/banshee,arfbtwn/banshee,arfbtwn/banshee,ixfalia/banshee,GNOME/banshee