compile: Print error messages on unimplemented relative imports.

py/compile.c

@@ -1261,11 +1261,13 @@ void do_import_name(compiler_t *comp, mp_parse_node_t pn, qstr *q1, qstr *q2) {
         } else {
             // TODO not implemented
             // This covers relative imports starting with dot(s) like "from .foo import"
+            compile_syntax_error(comp, "Relative imports not implemented");
             assert(0);
         }
     } else {
         // TODO not implemented
         // This covers relative imports with dots only like "from .. import"
+        compile_syntax_error(comp, "Relative imports not implemented");
         assert(0);
     }
 }