Merge branch 'bugfix/django-material-error-messages' into 'master'

Add custom templates for error messages in forms generated by django-material

See merge request AlekSIS/AlekSIS!147

aleksis/core/templates/material/field_errors.html

+{% if bound_field.errors %}
+  <div class="errors">
+    {% for error in bound_field.errors %}<small class="error-text">{{ error }}</small>{% endfor %}
+  </div>
+{% endif %}

aleksis/core/templates/material/non_field_errors.html

+{% if form.non_field_errors %}
+  <div class="alert error">
+    {% for error in form.non_field_errors %}
+      <div><i class="material-icons left">error</i> {{ error }}</div>
+    {% endfor %}
+  </div>
+{% endif %}