Add retries to InstallVisualStudio.ps1 script (#9300)
Add retries to InstallVisualStudio.ps1 script (#9300)

- short-term attempt to reduce CI problems in this area; see dotnet/core-eng#5760
- add `-ErrorAction Continue` to the `Start-Process` command line to enable retries

nits & debugging:
- display and interpret any exit code from the VS installer
- display VS installation logs
- remove unnecessary `[CmdletBinding]` attribute
- spell out `-ForegroundColor Red`
- add "Error: " prefix to error messages
- remove unnecessary empty strings passed to `Write-Host`
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