[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/tests/expression/errors/locals_in_globals.fine

@@ -0,0 +1,12 @@
+{
+  // This is a block-local declaration; it should *not* appear in the global
+  // environment.
+  let y = 23;
+}
+
+fun foo() -> f64 {
+  y + 3
+}
+
+// @expect-errors:
+// | 8:2: cannot find value y here