Display correct interface message when adding duplicate institution

This makes the interface display the correct warning message, renamed
<ep-addorg-exists-already>, when attemping to add an institution
with a name that already exists. It also refactors how these types of
message strings are created for institution page and course pages.

bug 56327

Change-Id: Ifc9f8fa46a99c9ff3d58b6662668180f18a0733e
Merge "Display correct interface message when adding duplicate institution"