Fix string-based error inspection on Python 3

Author: mwaskom

nipype/interfaces/io.py

@@ -71,7 +71,7 @@ def copytree(src, dst, use_hardlink=False):
     try:
         os.makedirs(dst)
     except OSError as why:
-        if 'File exists' in why:
+        if 'File exists' in str(why):
             pass
         else:
             raise why
@@ -687,7 +687,7 @@ def _list_outputs(self):
                 try:
                     os.makedirs(outdir)
                 except OSError as inst:
-                    if 'File exists' in inst:
+                    if 'File exists' in str(inst):
                         pass
                     else:
                         raise(inst)
@@ -738,7 +738,7 @@ def _list_outputs(self):
                         try:
                             os.makedirs(path)
                         except OSError as inst:
-                            if 'File exists' in inst:
+                            if 'File exists' in str(inst):
                                 pass
                             else:
                                 raise(inst)