feat(selfhost-sema): arithmetic errors return tyUnknown sentinel

_test_error_recovery/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "error_recovery"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"

_test_error_recovery/src/Main.bux

@@ -0,0 +1,5 @@
+func Main() -> int {
+    let bad: int = "a" + 1;
+    let z: int = undefined_variable;
+    return 0;
+}

src/sema.bux

@@ -966,7 +966,7 @@ module Sema {
             if op == tkAmpAmp || op == tkPipePipe || op == tkBang { return tyBool; }
             // Arithmetic returns wider type
             if !Sema_IsNumeric(left) || !Sema_IsNumeric(right) {
-                Sema_EmitError(sema, expr.line, expr.column, "arithmetic requires numeric operands");
+                return Sema_EmitExprError(sema, expr, "arithmetic requires numeric operands");
             }
             if left == tyFloat64 || right == tyFloat64 { return tyFloat64; }
             return tyInt;