[bindings] Pipe errors back from write_template_constructor

[bindings] Pipe errors back from write_template_constructor

We can fail to resolve a part of a tuple, resulting in a panic in
write_template_constructor even if we're calling
`understood_c_type` with the intent of figuring out whether we can
print a type at all. Instead, we should pipe errors back and let
`understood_c_type` return false as a result.