Metadata delete doesn't show reason for failure

[Xander Victory]

Trying to delete metadata (from Production in this case) can fail and not provide a reason:

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Deleted 30/30 components from BlackAndWhitePROD in 19 s 962 ms with status FAILED.

Successes
=========
• classes/AppConstants.cls
• classes/CustomMetadataLoaderController.cls
• classes/CustomMetadataLoaderControllerTest.cls
• classes/CustomMetadataUploadController.cls
• classes/CustomMetadataUploadControllerTest.cls
• classes/MetadataApexApiLoader.cls
• classes/MetadataLoader.cls
• classes/MetadataLoaderClient.cls
• classes/MetadataLoaderFactory.cls
• classes/MetadataMapper.cls
• classes/MetadataMapperCustom.cls
• classes/MetadataMapperDefault.cls
• classes/MetadataMapperFactory.cls
• classes/MetadataMapperSimple.cls
• classes/MetadataMapperType.cls
• classes/MetadataMappingInfo.cls
• classes/MetadataMigrationController.cls
• classes/MetadataMigrationException.cls
• classes/MetadataObjectCreator.cls
• classes/MetadataOpType.cls
• classes/MetadataResponse.cls
• classes/MetadataService.cls
• classes/MetadataServiceTest.cls
• classes/MetadataUtil.cls
• classes/MetadataWrapperApiLoader.cls
• pages/CMTMigrator.page
• pages/CustomMetadataLoader.page
• pages/CustomMetadataRecordUploader.page
• tabs/CMT_Migrator.tab-meta.xml
• tabs/Custom_Metadata_Loader.tab-meta.xml

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In this case, there was a Mock class that was missing from the selection, but I also saw the same behaviour when a test was failing, just the wait time was longer.

[Scott Wells]

Xander, 99% of the time when this happens, it's because either no additional error details were included in the response (less common), or the error details were jammed into the response in some unexpected location. If you can reproduce this, please do so with debug logging enables for both metadata removal/delete and, if this is in a source format project, for Salesforce DX so I can see the raw CLI output. If it's just a matter of looking in yet another location for error messages, that will easy to address.