MdeModulePkg/UefiBootManagerLib: log reserved mem allocation failure

MdeModulePkg/UefiBootManagerLib: log reserved mem allocation failure

The LoadFile protocol can report such a large buffer size that we cannot
allocate enough reserved pages for. This particularly affects HTTP(S)
Boot, if the remote file is very large (for example, an ISO image).

While the TianoCore wiki mentions this at
<https://github.com/tianocore/tianocore.github.io/wiki/HTTP-Boot#ram-disk-image-size>:

> The maximum RAM disk image size depends on how much continuous reserved
> memory block the platform could provide.

it's hard to remember; so log a DEBUG_ERROR message when the allocation
fails.

This patch produces error messages such as:

> UiApp:BmExpandLoadFile: failed to allocate reserved pages:
> BufferSize=4501536768
> LoadFile="PciRoot(0x0)/Pci(0x3,0x0)/MAC(5254001B103E,0x1)/
>      IPv4(0.0.0.0,TCP,DHCP,192.168.124.106,192.168.124.1,255.255.255.0)/
>      Dns(192.168.124.1)/
>      Uri(https://ipv4-server/RHEL-7.7-20190723.1-Server-x86_64-dvd1.iso)"
> FilePath=""

(Manually rewrapped here for keeping PatchCheck.py happy.)

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>