use exceptions for job cancel and failure

awx/main/exceptions.py

@@ -0,0 +1,24 @@
+class AwxTaskError(Exception):
+    """Base exception for errors in unified job runs"""
+    def __init__(self, task, message=None):
+        if message is None:
+            message = "Execution error running {}".format(task.log_format)
+        super(AwxTaskError, self).__init__(message)
+        self.task = task
+ 
+ 
+class TaskCancel(AwxTaskError):
+    """Canceled flag caused run_pexpect to kill the job run"""
+    def __init__(self, task, rc):
+        super(TaskCancel, self).__init__(
+            task, message="{} was canceled (rc={})".format(task.log_format, rc))
+        self.rc = rc
+
+
+class TaskError(AwxTaskError):
+    """Userspace error (non-zero exit code) in run_pexpect subprocess"""
+    def __init__(self, task, rc):
+        super(TaskError, self).__init__(
+            task, message="%s encountered an error (rc=%s), please see task stdout for details.".format(task.log_format, rc))
+        self.rc = rc
+