`generate.sh` should error gracefully when `meta.yml` does not exist

[kyoDralliam]

The generating scripts does not error when meta.yml does not exists but creates a 'Error: open meta.yml: no such file or directory.opam' file.

$ ~/(coq-community-template)> nix-shell -p mustache-go --run $TMP/generate.sh
this path will be fetched (1.10 MiB download, 3.83 MiB unpacked):
  /nix/store/ibnlmx9r9cxxhb2bvk7g3rhzhm2kc0dx-mustache-go-1.3.0
copying path '/nix/store/ibnlmx9r9cxxhb2bvk7g3rhzhm2kc0dx-mustache-go-1.3.0' from 'https://cache.nixos.org'...
Generating README.md...
mkdir: created directory '.circleci'
Generating .circleci/config.yml...
Generating Error: open meta.yml: no such file or directory.opam...
mkdir: created directory '.github'
mkdir: created directory '.github/workflows'
Generating .github/workflows/docker-action.yml...
Generating dune-project...
Generating theories/dune...
Generating Error: open meta.yml: no such file or directory.opam...
mkdir: created directory 'resources'
Generating resources/index.md...
Generating .github/workflows/nix-action.yml...
Generating .travis.yml...

$ ~/(coq-community-template) [1]> ls
 _CoqProject   'Error: open meta.yml: no such file or directory.opam'   Makefile       Makefile.coq.conf   resources          theories
 dune-project   LICENSE                                                 Makefile.coq   README.md

It may be more useful for the script to error and propose to create a basic meta.yml file to be completed.

[Zimmi48]

Related to #42.