feat(selfhost-cli): surface parser diagnostics — syntax errors no longer pass check silently

_test_parse_recovery/bux.toml

@@ -0,0 +1,4 @@
+[Package]
+Name    = "parse_recovery"
+Version = "0.1.0"
+Type    = "bin"

_test_parse_recovery/src/Main.bux

@@ -0,0 +1,18 @@
+func Good() -> int {
+    return 1;
+}
+
+func Bad() -> int {
+    let x: int = ;
+    return 0;
+}
+
+func AlsoBad() -> int {
+    let y: int = 5 + ;
+    let z: int = 6;
+    return y;
+}
+
+func AlsoGood() -> int {
+    return 2;
+}