Create a new client context with logic to handle service throttling.

New-ClientContextWithRetry.ps1

function New-ClientContextWithRetry
{
    [cmdletbinding()]
    param
    (
        [Parameter(Mandatory=$true, ParameterSetName = "CredentialComponents")]
        [Parameter(Mandatory=$true, ParameterSetName = "SharePointOnlineCredential")]
        [string]$ContextUrl,

        [Parameter(Mandatory=$true, ParameterSetName = "CredentialComponents")]
        [string]$UserName,

        [Parameter(Mandatory=$true, ParameterSetName = "CredentialComponents")]
        [System.Security.SecureString]$SecurePassword,

        [Parameter(Mandatory=$true, ParameterSetName = "SharePointOnlineCredential")]
        [Microsoft.SharePoint.Client.SharePointOnlineCredentials]$SharePointOnlineCredential,

        [Parameter(Mandatory=$false, ParameterSetName = "CredentialComponents")]
        [Parameter(Mandatory=$false, ParameterSetName = "SharePointOnlineCredential")]
        [int]$Delay = 10,

        [Parameter(Mandatory=$false, ParameterSetName = "CredentialComponents")]
        [Parameter(Mandatory=$false, ParameterSetName = "SharePointOnlineCredential")]
        [int]$RetryAttempts = 5
    )
    
    begin
    {
        $executions = 1
        $clientContext = $null
    }
    process
    {
        do
        {
            try
            {
                $clientContext = New-Object Microsoft.SharePoint.Client.ClientContext($ContextUrl)
                
                if( $PSCmdlet.ParameterSetName -eq "CredentialComponents" )
                {
                    $clientContext.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($UserName, $SecurePassword)
                }
                else
                {
                    $clientContext.Credentials = $SharePointOnlineCredential
                }

                $clientContext.Load($clientContext.Web)
                $clientContext.Load($clientContext.Site)
                $clientContext.ExecuteQuery()

                return $clientContext
            }
            catch [System.Net.WebException]
            {
                $response = $_.Exception.Response -as [System.Net.HttpWebResponse]
                $statusCode = [int]$response.StatusCode

                Write-Verbose "Full Exception Details: $($_.Exception)"
                Write-Verbose "Status Code: $statusCode"

                # 429 and 503 are throttling exceptions.  
                if( $response -and ($statusCode -eq 429 -or $statusCode -eq 503) )
                {
                    Write-Warning -Message "Execution attempt $executions was throttled, trying again in $Delay seconds."

                    $executions++
                    Start-Sleep -Seconds $Delay
                }
                # seeing erroneous 403 exceptions that require a retry
                elseif( $response -and $statusCode -eq 403 )                
                {
                    Write-Warning -Message "Execution attempt $executions was forbidden, trying again in $Delay seconds."

                    $executions++
                    Start-Sleep -Seconds $Delay
                }
                elseif( $response -and $statusCode -eq 404 )                
                {
                    Write-Warning -Message "A site with the URL $($ContextUrl) was not found in the tenant."
                    return $null
                }
                else                
                {
                    Write-Error -Message "An error occurred connecting to $($ContextUrl). Exception: $($_.Exception.Message)"
                    return $null
                }
            }
            catch
            {
                Write-Verbose "Full Exception Details: $($_.Exception)"

                if( $_.Exception.Message -match "Access to this Web site has been blocked")
                {
                    Write-Warning -Message "Cannot contact site at the specified URL $ContextUrl. Access to this Web site has been blocked." 
                    return $null
                }

                Write-Error -Message "$($MyInvocation.MyCommand.Name) - Unexpected execution exception: $($_.Exception.Message)"
                return $null
            }
        }
        while( $executions -le $RetryAttempts )
    }
    end
    {
    }
}