Fixed crash in Py2 environment with no .env file

Tapis CLI is not intended to require a .env file, but logic for exploring
likely paths where one might be found only caught the OSError raised by
python-dotenv under Python3, not the IOError raised under Python2. This
fix simply enumerates both Exceptions as acceptable means of signaling
an .env file was not found at the designated path.

Tested under Python2 and Python3

Closes #225

Author: mwvaughn

tapis_cli/settings/config.py

@@ -18,13 +18,13 @@ def find_config(filename='.env'):
         try:
             # Search from __file__ up to /
             env_file = find_dotenv(filename, raise_error_if_not_found=True)
-        except OSError:
+        except (IOError, OSError):
             # Search from current working directory
             try:
                 env_file = find_dotenv(filename=filename,
                                        raise_error_if_not_found=True,
                                        usecwd=True)
-            except OSError:
+            except (IOError, OSError):
                 # Fall back to $HOME
                 pass
     return env_file