exp init: clean up error messages

[dberenbaum]

From #7130:

$ dvc -V
2.9.2

$ git init
$ dvc init
$ dvc exp init python train.py
ERROR: unexpected error - [Errno 2] No such file or directory: 'params.yaml'

• Why is it unexpected error? It should be like “ERROR: hyperparameters file ‘params.yaml’ is missing”. 
 🤔 Why dvc exp init does not create it if it is required?

• Do we really need [Errno 2]? In what cases the code helps users? This makes the message longer and breaks human readability.

$ dvc exp run
ERROR: failed to reproduce 'dvc.yaml': [Errno 2] No such file or directory: '/Users/dmitry/src/test/myproj/data'

• full path. It is supposed to be relative. Why: 1) to make it short - the full path provides zero value in most of use cases; 2) it makes it hard to submit reports (some users are not comfortable to share their directories struture - See '/Users/dmitry/src/test/ - bc it might contain confidential info like user and project names.

• why dvc exp init checked params.yaml but didn’t check (or create) data/ & src/?

$ mkdir data
$ dvc exp run
ERROR: failed to reproduce 'dvc.yaml': [Errno 2] No such file or directory: '/Users/dmitry/src/test/myproj/src'

• the same questions - [Errno 2], Full path.

• why it was not checked in dvc init? Or was not created?

$ mkdir src
$ dvc exp run
…
python: can't open file 'train.py': [Errno 2] No such file or directory
ERROR: failed to reproduce 'dvc.yaml': failed to run: python train.py, exited with 2

• “exited with 2” Is it useful information? Where the exit code 2 belongs to? It might confuse users. Error code can be in debug errors not here.

• Another issue - ideally, we should have a reference to stage (a user might have multiple stages with the same or similar commands). The message might look like:
 ERROR: failed to execute stage ‘train’ from ‘dvc.yaml’

[skshetry]

“exited with 2” Is it useful information? Where the exit code 2 belongs to? It might confuse users. Error code can be in debug errors not here.

I disagree with this, exit codes are important, well that's how you know it's succeeded or failed (or crashed).

Do we really need [Errno 2]? In what cases the code helps users? This makes the message longer and breaks human readability.
$ dvc exp run
ERROR: failed to reproduce 'dvc.yaml': [Errno 2] No such file or directory: '/Users/dmitry/src/test/myproj/data'

This is unrelated to dvc exp init and out of scope to be honest for this. This is part of a larger issue with DVC and happens in all of the commands. When I brought this a year ago for #5387, there were disagreements on how to handle these errors (cc @efiop).