How do I trap a function error?

[Netrelemo]

I'm using PowerQuery to try get the contents of a list on 1,000 plus SharePoint sites. 

So I have retrieved the list of site URLs and constructed the list URL, and now want to get  the contents of each list, merged into one long aggregated list. 

So I have written a function, and I'm using that mostly just fine. The problem is that sometimes a particular site doesn't have the list I am looking for. It doesn't exist. And I cannot find a way to trap the error. 

Expression.Error: We cannot convert a value of type Table to type Function

My function returns a "table", andsometimes "error". If I try "replace errors" it can't because it's a table, not just a text or number. 

let
    Source = (SiteURL as any) => 
    let
        aSource = OData.Feed(SiteURL , null, [Implementation="2.0"]),
        "Removed Other Columns" = Table.SelectColumns(aSource,{ "CostTypeFree1"},MissingField.UseNull)
        "Distincted" = Table.Group(#"aSource", {"CostTypeFree1"}, {{"Count", each Table.RowCount(_), type number}}),
        "Removed Columns" = Table.RemoveColumns(Distincted,{"Count"}),
    in
        #"Removed Other Columns"
in
    Source

So... my question: How do I handle the fact that sometimes a list doesn't exist, and then return something useful.

I'm thinking maybe some sort of 
    Try this web request. If I get a 404 then "No" else "ok"

It would be so nice if I could just use something like this:

let
    response = Web.Contents(
        "https://api.census.gov/data/2018/pep/population?get=POP&for=state"
    ),
    json = Json.Document(response)
in
    json

 ... but unfortunately in a SSO MFA environment, such simple efficient solutions don't seem possible. 

Here's another attempt:

let
    Source = (theurl as any) => 
    let
        aTest = try OData.Feed(theurl, null, [Implementation="2.0"]),
        test = Table.AddColumn(aTest, "ListCheck", each if aTest [HasError] then "List doesnt exist" else "ok")
    in
        test
in
    Source


// An error occurred in the query. Expression.Error: We cannot convert a value of type Record to type Table.
// Details:
//     Value=[Record]
//     Type=[Type]

[mahoneypat · Accepted solution]

You need an otherwise to go with your try

aSource = try OData.Feed(SiteURL , null, [Implementation="2.0"]) otherwise null

If it is the Removed Other Columns step that generates the error, you could add a step to add a column with Table.RowCount( ) after aSource, and then filter out any rows where the table does not have >=1 rows.

Pat