[SIP-191] Proposal for Improving Import Error Logging for Dashboard/Chart ZIP Imports

## [SIP] Proposal for Improved Error Reporting During Dashboard/Chart Import Validation

### Motivation

Currently, when importing dashboards or charts using the Superset Import API, any validation failure or metadata mismatch inside the import bundle results in opaque error messages such as:
~~~
Import dashboard failed for an unknown reason
~~~

The server logs contain a generic stack trace like:

KeyError: 580

…but the API response and UI do not expose which part of the import bundle is invalid.

This makes it difficult to debug problems such as:
- Mismatched IDs in `filter_scopes`
- Missing chart references in dashboard metadata
- Broken or outdated metadata produced by older Superset versions
- Incorrect ZIP structure

Users must manually inspect YAML/JSON inside the ZIP—something that is error-prone and slow.

Improving the error messages will significantly reduce debugging time and make import flows more predictable for developers.

---

### Proposed Change

1. Add Proper Aggregated Validation Error Logging

The current code in `superset/commands/importers/v1/assets.py` catches a list of validation exceptions but logs almost nothing, emitting only:

Import dashboard failed for an unknown reason

This proposal introduces structured, explicit error logging for each validation exception.

### Proposed implementation

Below is a production-aligned rewrite of the snippet the author experimented with:

```python
# superset/commands/importers/v1/assets.py

import logging
logger = logging.getLogger(__name__)

if exceptions:
for exc in exceptions:
# Log each validation exception clearly with details
logger.error(
"Import validation error: %s | details: %s",
str(exc),
getattr(exc, "messages", None)
)

# Raise a domain-specific error containing the full list
raise CommandInvalidError(
"Error importing assets due to validation failures",
exceptions=exceptions,
)
```

Improvements over the experimental version

- Uses logger (Superset convention) instead of logging

- Clearer message formatting

- Includes both the exception itself and its .messages attribute (if any)

- Raises a more descriptive CommandInvalidError

- Matches the surrounding command architecture in other importers

## Return Descriptive Error Messages via API

Example API response:

~~~
{
"error": "ImportValidationError",
"errors": [
{
"message": "Chart ID 580 referenced in filter_scopes does not exist.",
"exception_type": "KeyError"
}
]
}
~~~

### New or Changed Public Interfaces
API Responses

Import-related endpoints will now return:

More detailed error messages

Structured lists of validation failures

This is backward compatible because:

The HTTP code remains the same

Existing clients that ignore details remain unaffected

CLI (superset import-*)

The CLI will print the same improved messages, giving developers immediate insight into metadata issues.

No CLI flags or config changes required.

### New dependencies

None

### Migration Plan and Compatibility
No migrations required.

### Rejected Alternatives

1. Only improving server logs, not API responses

Many users run imports in CI/CD pipelines or via automated scripts and do not have access to server logs.

Start in superset/commands/importers/v1/assets.py, reading the existing validation-exception handling and the surrounding importer command architecture. Trace how Superset Import API and superset import-* report failures; done means validation errors expose structured details for dashboard and chart imports without changing the HTTP status or requiring new dependencies.

Written by the indexing model from the issue text.