remove: redundant fastcgi intercept error directives from site config stubs

[yCodeTech]

The fastcgi_intercept_errors and error_page directives that were added in #26 are redundant in the site configs, they are not used by nginx. It is only the main valet.conf that nginx uses these directives from, all others are ignored in sites configs. Also the proxy_intercept_errors directive is not needed either since the fastcgi_intercept_errors directive will also do the same job for proxied sites.

• Removed proxy_intercept_errors on; and fastcgi_intercept_errors on; directives from all proxy and fastcgi locations in the nginx stubs: proxy.valet.conf, secure.proxy.valet.conf, secure.valet.conf, and unsecure.valet.conf.
• Deleted the custom error page configuration for HTTP 500, 502, 503, and 504 errors, including the internal /500.html location and references to VALET_ERROR_TEMPLATE_PATH, from all affected configuration stubs.