Improve "adb sideload" error reporting.

Rather than quietly fall through to the legacy code with no explanation,
say why the first connection failed, and that the second attempt is only
going to be relevant for folks trying to sideload to a pre-KitKit device.

Before:

  $ adb sideload mysid-ota-424425.zip
  opening 'mysid-ota-424425.zip'...
  connecting...
  falling back to older sideload method...
  error: closed

After:

  $ adb sideload mysid-ota-424425.zip
  adb: sideload connection failed: no devices/emulators found
  adb: trying pre-KitKat sideload method...
  adb: pre-KitKat sideload connection failed: no devices/emulators found

Also switch the legacy code to just read+write blocks rather than trying to
load the whole OTA package into RAM (which is in short supply on Win32).
This is probably of no practical use other than that it lets us report a
better error message (better than "terminate called after throwing an
instance of 'std::bad_alloc'").

Bug: https://issuetracker.google.com/37139736
Test: "adb sideload" with no device/a mysid/an angler
Change-Id: Ia568044aac61bd054301ebfd7fcd5ad5d6f3147a