MINOR: fd: report an error message when failing initial allocations

MINOR: fd: report an error message when failing initial allocations

When starting with a huge maxconn (say 1 billion), the only error seen
is "No polling mechanism available". This doesn't help at all to resolve
the problem. Let's add specific alerts for the failed mallocs. Now we can
get this instead:

  [ALERT] 286/154439 (23408) : Not enough memory to allocate 2000000033 entries for fdtab!

This may be backported as far as 2.0 as it helps debugging bad configurations.