Consistent error handling

Apply a single, predictable error-handling strategy across cmdlets and SDK wrappers:

• Make existence semantics consistent: if a method supports throwIfNotExists (or callers rely on it), every related “read/modify” path must honor it the same way (return null vs throw), and callers should not silently diverge between Get/With* operations.
• Align messages with cmdlet intent: for “New” vs “Set” flows, warnings must reflect what the cmdlet is trying to do:
  • New: warn when the What-If already exists.
  • Set: warn when the What-If does not exist.
• Use argument-focused exceptions for user input errors: when inputs are invalid or mutually exclusive, validate client-side and throw AzPSArgumentException (not generic InvalidOperationException), with clear text (including the expected format when relevant).
• Validate critical identifiers early + test both paths: for parameters like -ResourceId, validate format upfront and add scenario tests for both success and failure.
• Use -Force to control recovery/continuation: if an existing resource affects behavior, query it first; if it exists/missing and -Force is not set, warn and stop/confirm; if -Force is set, proceed.

Example pattern (existence + intent + -Force + consistent throw semantics):

PSDeploymentStackWhatIfResult existing = null;
try
{
    existing = sdk.GetResourceGroupDeploymentStackWhatIfResult(
        rgName, stackName, throwIfNotExists: false);
}
catch (Exception ex)
{
    // Only treat as non-fatal if the strategy explicitly allows graceful degradation.
    existing = null;
}

if (existing != null)
{
    // NEW cmdlet intent: warn when exists; Set cmdlet would invert this logic.
    if (!Force.IsPresent)
        throw new AzPSArgumentException($"What-If result '{stackName}' already exists. Use -Force to overwrite.");
}
else
{
    // SET cmdlet intent: warn when missing; NEW cmdlet proceeds.
    if (this.IsSetCmdlet && !Force.IsPresent)
        throw new AzPSArgumentException($"What-If result '{stackName}' not found. Use -Force to continue.");
}

This standard improves graceful degradation, error clarity, and reduces inconsistent behavior across cmdlets and SDK conversion layers.