Fix wrong file name in error message

[dbrumann]

Removes fallback to old default file name, when specified config-file is not found. Subsequently, the file loader will throw the CouldNotReadFileException.php for the expected filename.

Fixes #789

[dbrumann]

Assuming a project with the new depfile deptrac.yaml exists, this is now the output when running analyse with different options:

command	legacy warning	file warning
none	no	no
analyse deptrac.yaml	yes, because we use the old depfile argument	no
analyse depfile.yaml	yes, because we use the old depfile argument	File ".../depfile.yaml" cannot be read.
analyse --config-file=deptrac.yaml	no	no
analyse --config-file=depfile.yaml	no	File ".../depfile.yaml" cannot be read.

Assuming a project with the old depfile depfile.yaml exists, this is now the output when running analyse with different options:

command	legacy warning	file warning
none	yes (because old filename of the default file)	no
analyse deptrac.yaml	yes, because we use the old depfile argument	File ".../deptrac.yaml" cannot be read.
analyse depfile.yaml	yes, because we use the old depfile argument	no
analyse --config-file=deptrac.yaml	no	File ".../deptrac.yaml" cannot be read.
analyse --config-file=depfile.yaml	no	no

This check only looks for the used option and default filename, but does not look at the file content. We have checks for that as well, but the deprecation warnings might not be shown depending on the default error output settings. In other words, if you just change the filename from depfile.yaml to deptrac.yaml without making the structural changes (moving the parameters to the parameters section), the warning will disappear for now, but the command will fail once we remove the BC-layer.