Allow indent text to be passed in

Instead of forcing single space indenting allow
the indenting string to be passed in so that others
can provided their favorite indent (tabs, two spaces
or other...)

Change-Id: I2dfb3baeaecb8bed197429b591daed69eb8cc834

taskflow/exceptions.py

@@ -34,7 +34,7 @@ class TaskFlowException(Exception):
     def cause(self):
         return self._cause
 
-    def pformat(self, indent=2):
+    def pformat(self, indent=2, indent_text=" "):
         """Pretty formats a taskflow exception + any connected causes."""
         if indent < 0:
             raise ValueError("indent must be greater than or equal to zero")
@@ -45,7 +45,7 @@ class TaskFlowException(Exception):
                 # We'll add our own newlines on at the end of formatting.
                 if line.endswith("\n"):
                     line = line[0:-1]
-                lines.append((" " * indent_by) + line)
+                lines.append((indent_text * indent_by) + line)
             try:
                 lines.extend(_format(excp.cause, indent_by + indent))
             except AttributeError: