Do not fail if template file does not exist

[armsnyder]

This allows overriding specific template files and fallback on the defaults.

Before this change, the behavior was for the tool to fail with Error: ENOENT: no such file or directory if one of the template files in a custom templates directory does not exist.

Caveat: With this fix it will still error if the overridden template uses includeFile on a template file that is not overridden:

Error [Eta Error]: Could not find the template "./route-docs". Paths tried: /redacted/route-docs.eta

Example output:

✨ try to read templates from directory "/redacted"
❗❗❗ data contracts template not found in ./data-contracts.eta
Code generator will use the default template
❗❗❗ http client template not found in ./http-client.eta
Code generator will use the default template
❗❗❗ route types template not found in ./route-types.eta
Code generator will use the default template
❗❗❗ route name template not found in ./route-name.eta
Code generator will use the default template
☄️  start generating your typescript api

Related: #166

[js2me]

Hello @armsnyder !
Thank you for your contribution!