Any partitions not recognized by installer (e.g. lvm2, or unallocated) will cause the install to fail at line 409 of x86ArchitectureTemplate.py (and other locations if that test is commented out) even if they are not assigned a mount point. AFAIK, they *should* be ignored.
*** This bug has been marked as a duplicate of 146600 ***
see my comments on 146600. Apparently, according to that bug's reporter, this is not a duplicate because his complaint was about the displaying of the cause of the bug to the screen rather than the objectionable behavior, which was my issue.
This behavior is intentional. It keeps people who ignored the warning that the GTK frontend gives them from completely trashing their drives.