message: Add fallback URLs for OmniSharp and Mono downloads
diff: {
  "description": "Mono Runtime (Linux / x86)",
  "url": "https://omnisharpdownload.azureedge.net/ext/mono.linux-x86-5.2.0.104.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/mono.linux-x86-5.2.0.104.zip",
  "installPath": "./bin",
  "platforms": [
  "linux"
  {
  "description": "Mono Runtime (Linux / x64)",
  "url": "https://omnisharpdownload.azureedge.net/ext/mono.linux-x86_64-5.2.0.104.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/mono.linux-x86_64-5.2.0.104.zip",
  "installPath": "./bin",
  "platforms": [
  "linux"
  {
  "description": "Mono Runtime (macOS)",
  "url": "https://omnisharpdownload.azureedge.net/ext/mono.osx-5.2.0.104.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/mono.osx-5.2.0.104.zip",
  "installPath": "./bin",
  "platforms": [
  "darwin"
  {
  "description": "Mono Framework Assemblies",
  "url": "https://omnisharpdownload.azureedge.net/ext/framework-5.2.0.104.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/framework-5.2.0.104.zip",
  "installPath": "./bin/framework",
  "platforms": [
  "darwin",
  {
  "description": "OmniSharp (.NET 4.6 / x86)",
  "url": "https://omnisharpdownload.azureedge.net/ext/omnisharp-win-x86-1.21.0.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/omnisharp-win-x86-1.21.0.zip",
  "installPath": "./bin/omnisharp",
  "platforms": [
  "win32"
  {
  "description": "OmniSharp (.NET 4.6 / x64)",
  "url": "https://omnisharpdownload.azureedge.net/ext/omnisharp-win-x64-1.21.0.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/omnisharp-win-x64-1.21.0.zip",
  "installPath": "./bin/omnisharp",
  "platforms": [
  "win32"
  {
  "description": "OmniSharp (Mono 4.6)",
  "url": "https://omnisharpdownload.azureedge.net/ext/omnisharp-mono-1.21.0.zip",
  + "fallbackUrl": "https://omnisharpdownload.blob.core.windows.net/ext/omnisharp-mono-1.21.0.zip",
  "installPath": "./bin/omnisharp",
  "platforms": [
  "darwin",
  