BUG 11810: VTK failed to compile with Tcl 8.6 due to errorLine.

The errorLine member of the Tcl interp struct is private in Tcl 8.6.

Change-Id: I8695e4a9853e87e613cdb9b8d0cb5d9b9437531b
Merge topic '11810-tcl-geterror' into master

792a0602 BUG 11810: VTK failed to compile with Tcl 8.6 due to errorLine.