e2fsck: Avoid scary failure messages on low-memory systems

e2fsck: Avoid scary failure messages on low-memory systems

On a very low-memory system, where ext2fs_check_desc() fails because
it can't allocate a block bitmap, catch this error and report it
immediate.  This avoids something like this, which could scare and
mislead the user:

   e2fsck: Group descriptors look bad... trying backup blocks...
   Media was not cleanly unmounted, check forced.
   Pass 1: Checking inodes, blocks, and sizes
   Error allocating block bitmap (1): Memory allocation failed
   e2fsck: aborted

Addresses-Debian-Bug: #509529

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>