Fix confusing ReadError traceback

Avoids confusing tracebacks when ReadErrors exceptions are raised
from the yaml reader.

Now we do print the filename that failed to load and stop processing
in order to avoid additional exceptions.

Also adds tests to prevent future regressions.

Fixes: #261

tests/test_syntax_errors.py

@@ -92,3 +92,8 @@ def test_sets(self):
                    '    a:\n'
                    '      set\n'
                    '...\n', None)
+
+    def test_non_printable_characters(self):
+        self.check('output: |\n'
+                   '    ERROR: text with ANSI escapes)',
+                   None, problem=(22, 0))

yamllint/linter.py

@@ -183,6 +183,12 @@ def get_syntax_error(buffer):
                               'syntax error: ' + e.problem + ' (syntax)')
         problem.level = 'error'
         return problem
+    except yaml.reader.ReaderError as e:
+        problem = LintProblem(e.position + 1,
+                              0,
+                              'syntax error: ' + e.reason + ' (syntax)')
+        problem.level = 'error'
+        return problem
 
 
 def _run(buffer, conf, filepath):
@@ -197,6 +203,12 @@ def _run(buffer, conf, filepath):
     # right line
     syntax_error = get_syntax_error(buffer)
 
+    # syntax errors include failure to load the file and we should not do
+    # any additional processing after if one is encountered.
+    if syntax_error:
+        yield syntax_error
+        return
+
     for problem in get_cosmetic_problems(buffer, conf, filepath):
         # Insert the syntax error (if any) at the right place...
         if (syntax_error and syntax_error.line <= problem.line and
@@ -215,9 +227,6 @@ def _run(buffer, conf, filepath):
 
         yield problem
 
-    if syntax_error:
-        yield syntax_error
-
 
 def run(input, conf, filepath=None):
     """Lints a YAML source.