Fallback to pure-python loader for better error when YAML loading fails (#6081)

tests/unit_tests/fixtures/yaml_util/broken_includetest.yaml

@@ -0,0 +1,18 @@
+---
+substitutions:
+  name: original
+
+wifi: !include
+  file: includes/broken_included.yaml.txt
+  vars:
+    name: my_custom_ssid
+
+esphome:
+  # should be substituted as 'original',
+  # not overwritten by vars in the !include above
+  name: ${name}
+  name_add_mac_suffix: true
+  platform: esp8266
+  board: !include {file: includes/scalar.yaml, vars: {var1: nodemcu}}
+
+  libraries: !include {file: includes/list.yaml, vars: {var1: Wire}}

tests/unit_tests/fixtures/yaml_util/includes/broken_included.yaml.txt

@@ -0,0 +1,5 @@
+---
+# yamllint disable-line
+   ssid: ${name}
+# yamllint disable-line
+       fdf: error