fail when the C compiler fails, and clean up when we do

compile-to-file returned process-wait's values and main dropped them,
so cc errors were printed, but then main compiler exited 0.

Also cleanup tmp C files when compilation failed.

tests/exit-code/nested-pointer.sex

@@ -0,0 +1,4 @@
+;;; Rejected by the writer, so emit-c throws: there is a temporary .c
+;;; by then, and it must not survive.
+(fn f () void
+  (var p (* (* char))))