chore(ffi): run gen_header.sh in CI environment (#2488)

Clean up the script so that any unexpected error terminates the
script, and stop suppressing errors that may contain useful
information (for example, that you are using the stable version but
need to use the nightly).

This is useful because if hyper.h is not up to date going forward the
CI should flag it. As is, there are a bunch of changes to hyper.h that
have not been checked in (or were generated by a newer version of the
cbindgen script.)

Fixes #2483.

capi/cbindgen.toml

@@ -1,4 +1,10 @@
+# See https://github.com/eqrion/cbindgen/blob/master/docs.md#cbindgentoml for
+# a list of possible configuration values.
 language = "C"
+header = """/*
+ * Copyright 2021 Sean McArthur. MIT License.
+ * Generated by gen_header.sh. Do not edit directly.
+ */"""
 include_guard = "_HYPER_H"
 no_includes = true
 sys_includes = ["stdint.h", "stddef.h"]