ade start silently ignore --rc flag if the file is not found

Motivation

ade should not silently ignore a non-existing rc file. If the file is not existing, ade should report an error.

If the user uses a non-existing rc file by mistake, the user will enter in a different ade environment.

When calling ade with the following line:

$ ade --rc .aderc_env1

ade start will load the enviornment variables from file .aderc_env1 if this is found in the current or in an ancestor directory.

$ ade --rc .aderc_env1
INFO: Using non-default .aderc_env1 configuration file 
Entering desktop with following images:
...

However, if the file does not exist in the current or in an ancestor, the same output message will be shown:

$ ade --rc .aderc_non_existing
INFO: Using non-default .aderc_non_existing configuration file 
Entering desktop with following images:
...

but, instead of entering the environment requested by the user, ade will enter in the default environment (ade) if this is active.

I would suggest to fail and report an error if the requested .aderc file is not found, or at least to warn the user.

The fail/warning behaviour can be added in dotenv.py

If the suggested behaviour is accepted, I can create an MR.