Improve error when a group was not added to the lockfile yet

[matthid]

Description

Script failed
-> KeyNotFoundException: The given key was not present in the dictionary.
   StackTrace:
        at Microsoft.FSharp.Collections.MapTreeModule.find[TValue,a](IComparer`1 comparer, TValue k, MapTree`2 m)
        at Paket.LoadingScripts.ScriptGeneration.generateScriptContent@249-2.Invoke(GroupName group)
        at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
        at Paket.LoadingScripts.ScriptGeneration.generateScriptContent(PaketContext ctx)
        at Fake.Runtime.FakeRuntime.writeIntellisenseFile@198(VerboseLevel logLevel, String cacheDir, PaketContext context) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 202
        at Fake.Runtime.FakeRuntime.restoreOrUpdate@229(String script, VerboseLevel logLevel, String cacheDir, Dependencies paketDependencies, String groupStr, GroupName groupName, FrameworkIdentifier framework, FileInfo lockFilePath, Unit unitVar0) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 298
        at Fake.Runtime.FakeRuntime.paketCachingProvider(String script, VerboseLevel logLevel, String cacheDir, Dependencies paketDependencies, FSharpOption`1 group) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 352
        at Fake.Runtime.FakeRuntime.prepareFakeScript(FSharpList`1 defines, VerboseLevel logLevel, String script) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 453
        at Fake.Runtime.FakeRuntime.prepareAndRunScriptRedirect(VerboseLevel logLevel, FSharpList`1 fsiOptions, String scriptPath, FSharpList`1 scriptArgs, FSharpFunc`2 onErrMsg, FSharpFunc`2 onOutMsg, Boolean useCache) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 483
        at Fake.Runtime.FakeRuntime.prepareAndRunScript(VerboseLevel logLevel, FSharpList`1 fsiOptions, String scriptPath, FSharpList`1 scriptArgs, Boolean useCache) in C:\proj\FAKE\src\app\Fake.Runtime\FakeRuntime.fs:line 500
        at Program.runOrBuild(RunArguments args) in C:\proj\FAKE\src\app\Fake.netcore\Program.fs:line 104