feat: support fallible results and scoped cleanup

source/diagnostics/diagnostics.hon

@@ -99,6 +99,14 @@ Code :: enum {
     sentinel_mismatch
     overlapping_memory_copy
     unsupported_foreign_call
+    invalid_fallible_type
+    invalid_sum_type
+    invalid_fallible_operand
+    try_outside_fallible
+    incompatible_propagated_error
+    catch_type_mismatch
+    errdefer_outside_fallible
+    cleanup_control_flow
 }
 
 Diagnostic :: struct {
@@ -219,5 +227,13 @@ messages :: $enummap.init(Code, []u8, {
     sentinel_mismatch = "slice sentinel does not match the expected value",
     overlapping_memory_copy = "memory copy source and destination overlap",
     unsupported_foreign_call = "foreign function calls are unsupported by the interpreter",
+    invalid_fallible_type = "fallible type requires a concrete success type and an enum, struct, or tagged union error channel",
+    invalid_sum_type = "sum composition requires compatible named enum, struct, or tagged union variants",
+    invalid_fallible_operand = "try and catch require a fallible operand",
+    try_outside_fallible = "try requires an enclosing fallible function",
+    incompatible_propagated_error = "propagated errors are not a subset of the function error channel",
+    catch_type_mismatch = "catch handler must produce the success type or terminate",
+    errdefer_outside_fallible = "errdefer requires an enclosing fallible function",
+    cleanup_control_flow = "cleanup cannot escape with return, break, continue, or try",
 })