Single-file: Avoid exception-throw in startup path (dotnet/core-setup#6598)

Single-file: Avoid exception-throw in startup path (dotnet/core-setup#6598)

This commit implements two changes:

1) If the current AppHost is not a bundle, process_manifest_footer() returns a bool value
   to indicate the same, rather than throwing StatusCode::AppHostExeNotBundle.
   This ensures that we do not throw every time a non-bundled app-host runs, which
   is undesirable (especially while debugging).

2) If a single-file app is simultaneously started multiple times, the apphost begins
   extracting files for each instantiation. However, once one of the runs complete the
   extraction process, other runs will use the files extracted to the committed-directory.
   The AppHost tries to clean up the redundantly extracted files and directories.
   However, if there is an I/O error while doing so, the process is now allowed
   to continue execution, instead of failing.

Commit migrated from https://github.com/dotnet/core-setup/commit/bd497d99a0f029ebed76e5398b1db769c4cc0d3c