Added a little extra info to two bmpman errors.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11160 387891d4-d844-0410-90c0-e4c51a9137d3
Clarified how to set a new variable with mn.SEXPVariables.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11161 387891d4-d844-0410-90c0-e4c51a9137d3