Misleading error when running shore in folder that doesn't have main.pipeline.jsonnet

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### Description

Original Author: @slonokot

``` shell
shore render
env variable `SHORE_PROJECT_PATH` is not set
```

### Steps to Reproduce

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**Expected behavior:**

**Actual behavior:**

**Reproduces how often:**

### Versions

### Additional Information

Reproduce the issue by running `shore render` in a folder without `main.pipeline.jsonnet`; the payload reports the current `SHORE_PROJECT_PATH` error. Then locate the CLI path that handles a missing project file and clarify the expected error behavior before changing it. Done means the command reports the missing project configuration accurately.

Written by the indexing model from the issue text.