Better error messages (#445)

* wip

* overhauled yaml validation error messages

* use yaml client (mostly) everywhere

* fix imports

* fix yaml client namespace for python2

* pep8

* code cleanup + typos


automatic commit by git-black, original commits:
  e9177e2

core/dbt/clients/yaml_helper.py

@@ -26,14 +26,12 @@ def line_no(i, line, width=3):
 
 
 def prefix_with_line_numbers(string, no_start, no_end):
- line_list = string.split('\n')
+ line_list = string.split("\n")
 
  numbers = range(no_start, no_end)
  relevant_lines = line_list[no_start:no_end]
 
- return "\n".join([
- line_no(i + 1, line) for (i, line) in zip(numbers, relevant_lines)
- ])
+ return "\n".join([line_no(i + 1, line) for (i, line) in zip(numbers, relevant_lines)])
 
 
 def contextualized_yaml_error(raw_contents, error):
@@ -44,9 +42,9 @@ def contextualized_yaml_error(raw_contents, error):
 
  nice_error = prefix_with_line_numbers(raw_contents, min_line, max_line)
 
- return YAML_ERROR_MESSAGE.format(line_number=mark.line + 1,
-  nice_error=nice_error,
-  raw_error=error)
+ return YAML_ERROR_MESSAGE.format(
+ line_number=mark.line + 1, nice_error=nice_error, raw_error=error
+ )
 
 
 def safe_load(contents) -> Optional[Dict[str, Any]]:
@@ -57,7 +55,7 @@ def load_yaml_text(contents):
  try:
  return safe_load(contents)
  except (yaml.scanner.ScannerError, yaml.YAMLError) as e:
- if hasattr(e, 'problem_mark'):
+ if hasattr(e, "problem_mark"):
  error = contextualized_yaml_error(contents, e)
  else:
  error = str(e)