Once variable is assigned a value do not let it be overwritten - excel

Question

I have a global variable ErrorMsg that can be set based on multiple different functions. The problem I am running into is if a certain condition is met, then the macro will assign ErrorMsg a text string that will be displayed in the worksheet Log Sheet. The way I have my code set up, even if an error occurs and ErrorMsg has a set text string assigned to it, the macro will continue running all the way through all the data until it is at the end. The issue with this that a lot of the conditions stem off each other. So if the function GetColumnIndex throws an error, assigns ErrorMsg a text string and continues on, then the next function like GetData will also throw an error and overwrite the value of ErrorMsg to be whatever it is set within its function. This occurs because the value that comes from GetColumnIndex is an input in GetData function. So my question is, how do I tell the macro to recognize once a value has been set to ErrorMsg to continue running through the code, but to not overwrite the variable with an updated text string. As you can see in the script, each function has a line that defines what ErrorMsg is. I am looking to find a way for if GetColumnIndex set ErrorMsg to be the text string to not have GetData overwrite ErrorMsg to a different text string.
Here are the functions I mentioned above.
Global ErrorMsg As String
Sub Main
Dim cell As Range, ws As Worksheet, sysnum As String, sysrow As Integer, wb As Workbook, logsht As Worksheet
Dim power_col As Long, power_value As Double
Set wb = ActiveWorkbook
Set ws = ActiveWorksheet
Set logsht = wb.Worksheets("Log Sheet")
For Each cell In ws.Range("E2", ws.cells(ws.Rows.Count, "E").End(xlUp)).cells
sysnum = cell.Value
sysrow = cell.row
power_col = GetColumnIndex(ws, "Power (mW)")
power_value = GetJiraData(ws, sysrow, power_col)
Dim begincell As Long
With logsht
begincell = .cells(Rows.Count, 1).End(xlUp).row
.cells(begincell + 1, 2).Value = sysnum
.cells(begincell + 1, 2).Font.Bold = True
If Not ErrorMsg = "" Then
.cells(begincell + 1, 3).Value = "Complete with Erorr - " & ErrorMsg
.cells(begincell + 1, 3).Font.Bold = True
.cells(begincell + 1, 3).Interior.Color = vbRed
Else
.cells(begincell + 1, 3).Value = "Completed without Errors"
.cells(begincell + 1, 3).Font.Bold = True
.cells(begincell + 1, 3).Interior.Color = vbGreen
End If
End With
Next cell
End Sub
Function GetColumnIndex(sht As Worksheet, colname As String) As Double
Dim paramname As Range
Set paramname = sht.Range("A1", sht.cells(2, sht.Columns.Count).End(xlToLeft)).cells.Find(What:=colname, Lookat:=xlWhole, LookIn:=xlFormulas, searchorder:=xlByColumns, searchdirection:=xlPrevious, MatchCase:=True)
If Not paramname Is Nothing Then
GetColumnIndex = paramname.Column
ElseIf paramname Is Nothing Then
ErrorMsg = colname & " column index could not be found. Check before running again."
End If
End Function
Function GetData(sht As Worksheet, WDrow As Integer, parametercol As Long)
GetData = sht.cells(WDrow, parametercol)
If GetData = -999 Then
ElseIf GetData < 0 Then
ErrorMsg = "Data cannot be a negative number. Check before running again."
End If
End Function

Answer 1

You can use
If lenb(ErrorMsg) > 0 then ErrorMsg = ErrorMsg & vbCrLf`
ErrorMsg = ErrorMsg & "your text"
And when you have written the error message to the sheet, clear ErrorMsg:`
ErrorMsg = vbNullString