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>

tests/test_interrupts.asm

@@ -1,9 +1,15 @@
 .section code 0x1000 {
     main:
         ; Test that interrupts will not called when enabled flag is not set
-        int 0, 0
+        ; (using 0x80 because anything below it is an error interrupt, and
+        ;  invoking an error interrupt when disabled will cascade to a triple
+        ;  fault)
+        mov %status, 1
+        int 0x80, 0
         cmpeq (count), 0
-        jnz end
+        jz end
+
+        or %flags, 0b100
 
         ; Test divide by zero interrupts
 
@@ -20,6 +26,16 @@
         cmpeq (count), 2
         jz end
 
+        ; Test illegal memory address interrupts
+        add %status, 1
+        ; TODO - fix this bug, this line breaks because it's trying to
+        ; calculate an address that's too large without using wrapping addition
+        ; functions
+        ;mov %status, (0xfffffffffffffff)
+        mov %status, (0xffffffff)
+        cmpeq (count), 3
+        jz end
+
         mov %status, 0
     end:
         halt
@@ -37,7 +53,7 @@
     ivt:
     .interrupt 0, 0
     .interrupt 0, 0
-    .interrupt 0, 0
+    .interrupt 1, generic_handler
     .interrupt 1, generic_handler
     .export ivt
 }