Don't treat non-importable YAML as syntax error

`yaml.load()` exceptions are not necessarily syntax errors. For
instance, the following YAML source cannot be `load()`ed into a Python
object, but is valid nonetheless:

    ? - Detroit Tigers
      - Chicago cubs
    :
      - 2001-07-23

    ? [ New York Yankees,
        Atlanta Braves ]
    : [ 2001-07-02, 2001-08-12,
        2001-08-14 ]

This commit detects syntax errors from `yaml.parse()` exceptions rather
than `yaml.load_all()`.

tests/rules/test_syntax_error.py

@@ -35,3 +35,17 @@ class YamlLintTestCase(RuleTestCase):
                    '%TAG ! tag:clarkevans.com,2002:\n'
                    'doc: ument\n'
                    '...\n', None, problem=(3, 1))
+
+    def test_mapping_between_sequences(self):
+        # This is valid YAML. See http://www.yaml.org/spec/1.2/spec.html,
+        # example 2.11
+        self.check('---\n'
+                   '? - Detroit Tigers\n'
+                   '  - Chicago cubs\n'
+                   ':\n'
+                   '  - 2001-07-23\n'
+                   '\n'
+                   '? [New York Yankees,\n'
+                   '   Atlanta Braves]\n'
+                   ': [2001-07-02, 2001-08-12,\n'
+                   '   2001-08-14]\n', None)

yamllint/__init__.py

@@ -58,7 +58,7 @@ def get_costemic_problems(buffer, conf):
 
 def get_syntax_error(buffer):
     try:
-        list(yaml.safe_load_all(buffer))
+        list(yaml.parse(buffer))
     except yaml.error.MarkedYAMLError as e:
         problem = LintProblem(e.problem_mark.line + 1,
                               e.problem_mark.column + 1,