`Get-ChangelogData`: Throws an exception if `Unreleased` header is misspelled

[johlju]

This line throws an error if the Unreleased is misspelled.

https://github.com/natescherer/ChangelogManagement/blob/7da86a37f98f9ecdf822c5cc02f86a7cd60dc979/src/public/Get-ChangelogData.ps1#L113

Having the change log like:

## [Unrelesed]

## [0.12.0] - 2022-12-10

### Added

- Something added

Calling this:

Get-ChangelogData -Path .\CHANGELOG.md

throws this:

Exception             :
    Type        : System.Management.Automation.RuntimeException
    ErrorRecord :
        Exception             :
            Type    : System.Management.Automation.ParentContainsErrorRecordException
            Message : You cannot call a method on a null-valued expression.
            HResult : -2146233087
        CategoryInfo          : InvalidOperation: (:) [], ParentContainsErrorRecordException
        FullyQualifiedErrorId : InvokeMethodOnNull
        InvocationInfo        :
            ScriptLineNumber : 113
            OffsetInLine     : 25
            HistoryId        : -1
            ScriptName       : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1
            Line             :             "Date"    = Get-Date ($Release -split "\] \- ")[1].Split($FileNewline)[0]

            PositionMessage  : At C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1:113 char:25
                               + … "Date"    = Get-Date ($Release -split "\] \- ")[1].Split($FileNewline …
                               +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            PSScriptRoot     : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public
            PSCommandPath    : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1
            CommandOrigin    : Internal
        ScriptStackTrace      : at Get-ChangelogData, C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1: line 113
                                at <ScriptBlock>, <No file>: line 1
    TargetSite  : System.Object CallSite.Target(System.Runtime.CompilerServices.Closure, System.Runtime.CompilerServices.CallSite, System.Object, System.Object)
    Message     : You cannot call a method on a null-valued expression.
    Source      : Anonymously Hosted DynamicMethods Assembly
    HResult     : -2146233087
    StackTrace  :
   at CallSite.Target(Closure, CallSite, Object, Object)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
   at lambda_method2941(Closure, Object[], StrongBox`1[], InterpretedFrame)
CategoryInfo          : InvalidOperation: (:) [], RuntimeException
FullyQualifiedErrorId : InvokeMethodOnNull
InvocationInfo        :
    ScriptLineNumber : 113
    OffsetInLine     : 25
    HistoryId        : -1
    ScriptName       : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1
    Line             :             "Date"    = Get-Date ($Release -split "\] \- ")[1].Split($FileNewline)[0]

    PositionMessage  : At C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1:113 char:25
                       + … "Date"    = Get-Date ($Release -split "\] \- ")[1].Split($FileNewline …
                       +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    PSScriptRoot     : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public
    PSCommandPath    : C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1
    CommandOrigin    : Internal
ScriptStackTrace      : at Get-ChangelogData, C:\source\DscResource.Common\output\RequiredModules\ChangelogManagement\3.0.0\public\Get-ChangelogData.ps1: line 113
                        at <ScriptBlock>, <No file>: line 1

[natescherer]

Having some level of document structure validation is probably a good idea rather than just assuming the doc matches KaC 1.0.0. I'll throw the general concept of document validation for Update-Changelog and Get-ChangelogData onto the todo pile.

[johlju]

Yeah, I think it just need to throw a better error message saying that header X does not conform to the expected format.