Omnisharp fails to install on a flaky network

## Issue Description ##
When on a flaky network the download procedure seems to silently ignore download failures and blindly proceeds to run the installation, which fails and Omnisharp complains about missing files.

## Steps to Reproduce ##

1. Be on a flaky network
2. Install the extension

## Expected Behavior ##
Installation succeeds and extension starts up.

## Actual Behavior ##
Installation fails

## Logs ##
Curl samples

gtaillon@m1:~/.vscode-server/extensions$ curl -LO https://roslynomnisharp.blob.core.windows.net/releases/1.37.3/omnisharp-linux-x64-1.37.3.zip
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
72 46.2M 72 33.2M 0 0 1581k 0 0:00:29 0:00:21 0:00:08 0
curl: (18) transfer closed with 13533251 bytes remaining to read
gtaillon@m1:~/.vscode-server/extensions$ curl -LO https://roslynomnisharp.blob.core.windows.net/releases/1.37.3/omnisharp-linux-x64-1.37.3.zip
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
29 46.2M 29 13.5M 0 0 2121k 0 0:00:22 0:00:06 0:00:16 3170k
curl: (56) OpenSSL SSL_read: error:1408F119:SSL routines:ssl3_get_record:decryption failed or bad record mac, errno 0
gtaillon@m1:~/.vscode-server/extensions$ curl -LO https://roslynomnisharp.blob.core.windows.net/releases/1.37.3/omnisharp-linux-x64-1.37.3.zip
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
41 46.2M 41 19.2M 0 0 1077k 0 0:00:43 0:00:18 0:00:25 0
curl: (18) transfer closed with 28246083 bytes remaining to read
gtaillon@m1:~/.vscode-server/extensions$ curl --retry 100 -LO https://roslynomnisharp.blob.core.windows.net/releases/1.37.3/omnisharp-linux-x64-1.37.3.zip
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 46.2M 100 46.2M 0 0 3218k 0 0:00:14 0:00:14 --:--:-- 3578k

### OmniSharp log ###
Starting OmniSharp server at 11/1/2020, 2:15:18 PM
Target: /home/gtaillon/AribaIntegrations/Cae.Digital.AribaIntegration.sln

[ERROR] Error: spawn /home/gtaillon/.vscode-server/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.3/run ENOENT

### C# log ###
Installing C# dependencies...
Platform: linux, x86_64, name=debian, version=10

Downloading package 'OmniSharp for Linux (x64)' (47313 KB)......... Retrying from 'https://roslynomnisharp.blob.core.windows.net/releases/1.37.3/omnisharp-linux-x64-1.37.3.zip' (47313 KB)

## Environment information ##

**VSCode version**: 1.50.1
**C# Extension**: 1.23.4

Mono Information
OmniSharp using built-in mono
Dotnet Information
.NET Core SDK (reflecting any global.json):
Version: 3.1.403
Commit: 9e895200cd

Runtime Environment:
OS Name: debian
OS Version: 10
OS Platform: Linux
RID: debian.10-x64
Base Path: /usr/share/dotnet/sdk/3.1.403/

Host (useful for support):
Version: 3.1.9
Commit: 774fc3d6a9

.NET Core SDKs installed:
2.2.402 [/usr/share/dotnet/sdk]
3.1.403 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.8 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.8 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.9 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.8 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.9 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|azure-account|ms-vscode|0.9.3|
|azurecli|ms-vscode|0.5.0|
|azurerm-vscode-tools|msazurermtools|0.13.0|
|csharp|ms-dotnettools|1.23.4|
|powershell|ms-vscode|2020.6.0|
|vscode-azurefunctions|ms-azuretools|1.0.0|
|vscode-dotnet-runtime|ms-dotnettools|0.1.2|