feat: add errorHandler if resource could not be loaded

- Fixes #7
- Fixes #6
- Add "errorHandler" config key
- Add the following error-handlers:
  - `warning`: prints warning if resource not found under 'assets'
  - `warning-remote`: prints warning, but tries RemoteGet as alternative
  - `error`: prints error if resource not found, fails build

layouts/partials/error-handler/error.html

@@ -0,0 +1 @@
+{{ errorf "image path './assets/%s' does not exist. please check the 'assets' directory." .img }}

layouts/partials/error-handler/warning-remote.html

@@ -0,0 +1,12 @@
+{{ $params := . }}
+{{ $url := .img | absURL }} 
+{{ warnf "image path './assets/%s' does not exist. trying remote get via %s" .img $url }}
+{{ $res := false }}
+{{ with $resource := resources.GetRemote $url }}
+    {{ with .Err }}
+        {{ warnf "could not get remotely: %s: %#v" .Error .Data }}
+    {{ else }}
+        {{ $res = merge $params (dict "img" $resource) }}
+    {{ end }}
+{{ end }}
+{{ return $res }}

layouts/partials/error-handler/warning.html

@@ -0,0 +1 @@
+{{ warnf "image path './assets/%s' does not exist. please check the 'assets' directory." .img }}