Fixed: Improved exception message on failed schema import

[DebottamMandal123]

Fixes #8656 Improves exception handling during schema import operations to provide clearer and more actionable error messages.

Summary of Changes

1. ECDb Error Improvements (core/backend/src/ECDb.ts):

• Added pre-validation for schema file existence (fs.existsSync / fs.accessSync)

• Throw structured IModelStatus.FileNotFound errors including file path context

• Wrapped importSchema native call with contextual exception handling

2. IModelDb Enhancements (core/backend/src/IModelDb.ts):

• Introduced createSchemaImportError helper for consistent error messaging

• Pre-validation for imported schema file paths

• Improved context propagation for:

      - File load failures
  
      - XML import failures
  
      - Schema sync failures
  

3. Regression Fix (core/backend/src/test/hubaccess/Rebase.test.ts):

• Corrected incorrect usage of importSchemas in existing test

• Now writes schema XML to a temp file before import

4. Added New Tests:

• New suite: core/backend/src/test/standalone/SchemaImportErrors.test.ts

-Covers:

      - Missing file path → FileNotFound

      - Invalid schema content → detailed contextual error message

Validation:

1. Automated:

     - rush build 

     - Test suites (new + updated) —  All passed

Manual:

     - Verified clear messaging on missing file scenarios

📸 Screenshot of test results

[CLAassistant]

All committers have signed the CLA.

[kabentley]

I suggest you instead use this opportunity to introduce a new ITwinError interface for schema related errors instead of propagation the ErrorNumber insanity.