Extend how interrupts are reported to the main execution loop

Some things that were previously hard VM-level errors are now handled by
interrupts. While this is relatively easy to handle, I was wanting a
little more structure for the error types - so, errors that should
invoke an interrupt are passed along in their own structure in a
VmError::Interrupt variant. If an error is raised during the tick()
phase of execution that would cause an interrupt, that interrupt is
intercepted and the VM continues.

The State::interrupt() function also will catch double faults and triple
faults, with a triple fault being its own variant in the VmError
structure (so it cannot be intercepted as an interrupt by accident).

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

vm.md

@@ -343,12 +343,12 @@ will unconditionally halt the machine.
 
 * Double fault
     * Interrupt vector: 0x00
-    * Auxiliary: 
+    * Auxiliary: The interrupt vector that was being invoked that caused the fault.
     * An error state interrupt occurred while already handling an error state interrupt
-* Invalid opcode
+* Illegal instruction
     * Interrupt vector: 0x01
-    * Auxiliary: N/A
-    * Attempted to invoke an illegal opcode
+    * Auxiliary: Memory address where illegal instruction is located
+    * Attempted to execute a malformed instruction
 * Illegal memory address
     * Interrupt vector: 0x02
     * Auxiliary: Memory address causing the interrupt