feat: error handling and staged rollback capability

[kristopherjturner]

Overview

Implement robust error handling across all conversion scripts with staged rollback capability. Any failure during conversion must leave the VM in a recoverable state, not in a partially-converted limbo.

Scope

• Staged checkpoint system: save VM state before each conversion step
• Rollback logic for each stage: MBR/GPT, Gen1/Gen2 firmware, BCD changes
• Structured error output with error codes, affected resource, and recommended remediation
• Error catalog documentation (known failure codes and resolution steps)
• Graceful handling of partial failures in batch conversion mode (continue/abort options)

Acceptance Criteria

• Every destructive operation preceded by a checkpoint save
• Rollback from any stage returns VM to pre-modification state
• Error output includes: error code, affected VM, stage, and recommended action
• Batch mode can continue processing remaining VMs after a single failure
• Error catalog covers all known failure modes

Dependencies

• Parent Epic: [Epic] VM Conversion Toolkit Full Build-Out #5
• Azure Local path scripts: feat: script refinement — Azure Local conversion path (01–05 scripts) #6
• Hyper-V path scripts: feat: script refinement — Hyper-V standalone conversion path #7

[kristopherjturner]

Tracked in Azure DevOps: https://dev.azure.com/hybridcloudsolutions/Azure%20Local%20VM%20Conversion%20Toolkit/_workitems/edit/185