I was getting tired of the "An error occurred with your upload. Please try again." message so I wrote this to help import my Fitbit TCX files to Garmin Connect.  
Here are some basic instructions for the non powershell saavy.  
  Author: Michael Abair
  Date: 2017-05-04
  Usage: 
         1.) Save ConvertFitbitTCXFilesToGarmin.ps1 file from the zip above to your C:\ folder (for simplicity's sake)
         2.) Click Start->Run->Powershell
         3.) Type CD C:\ into the powershell window and hit enter (this changes the directory to the C:\ where you saved the ConvertFitbitTCXFilesToGarmin.ps1 File)
         4.) Type . .\ConvertFitbitTCXFilesToGarmin.ps1  and hit enter (this is how powershell loads the functions from the ps1 file)
         ###  Notice that the above is period space period slash filename; 
         5.) Type Convert-AllTCXFilesInFolder("C:\tcx") and hit enter..  Replace C:\tcx\ with the folder that has all of your unconverted tcx files or create a c:\tcx and put all your files there
         6.) Your files will all be converted and saved to C:\ConvertedTCXFiles.  When you go to import on garmin connect just look for them there.
I hope this will help someone else throw out their fitbit and enjoy their new Garmin! :)