So... I wrecked my phone last night. :-(
Android's phone encryption feature has been around for aaaages, so I was confident when I started the encryption process last night. Unfortunately, something went wrong, and last night I discovered that everything was toast.
After a bit of digging, I found someone else with exactly the same problem!
"unable to boot into my phone as it sits at the boot screen. When I try to boot into TWRP, it asks for my password. [...] it appears to decrypt the partition and mount, but then while loading, TWRP shows a continuous stream of:
E:Error parsing XML file
errors until it just restarts."
Unfortunately, nobody responded to my own personal denvercoder9.
Long story short, I lost everything. But for a while, I thought my entire phone was bricked. The phone wasn't accessible in either Recovery mode (due to the XML parsing catastrophe) or Download mode - it wasn't accessible either via adb or via fastboot.
Eventually my mate @rendition pointed out that I could just boot into Recovery, then cancel the decryption step and wipe my phone from there. That allowed me to start again from a clean slate.
And I encrypted my phone right away, with no trouble at all!