…utput (#1565)

Extensive overhaul of our downloads handling and console output; @JavierMatosD and I have gone back and forth several times and yet kept introducing unintended bugs in other places, which led me to believe targeted fixes would no longer cut it.

Fixes many longstanding bugs and hopefully makes our console output for this more understandable:
* We no longer print 'error' when an asset cache misses but the authoritative download succeeds. This partially undoes #1541. It is good to print errors immediately when they happen, but if a subsequent authoritative download succeeds we need to not print those errors.
* We now always and consistently print output from x-script s at the time that actually happens. Resolves https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2300063
* We don't tell the user that proxy settings might fix a hash mismatch problem.
* We do tell the user that proxy settings might fix a download from asset cache problem.
* We now always tell the user the full command line we tried when invoking an x-script that fails.
* We don't crash if an x-script doesn't create the file we expect, or creates a file with the wrong hash.
* We now always print what we are doing *before* touching the network, so if we hang the user knows which server is being problematic. Note that this includes storing back to asset caches which we were previously entirely silent about except in case of failure.

Other changes:
* Removed debug output about asset cache configuration. The output was misleading / wrong depending on readwrite settings, and echoing to the user exactly what they said before we've interpreted it is not useful debug output. (Contrast with other `VcpkgPaths` debug output which tend to be paths we have likely changed from something a user said) 

Other notes:
* This makes all dependencies of #908 speak `DiagnosticContext` so it will be easy to audit that the foreground/background thread behavior is correct after this.
* I did test the curl status parsing on old Ubuntu again.

Special thanks to @JavierMatosD for his help in review of the first console output attempts and for blowing the dust off this area in the first place.