IQA-463: Add error logging for certain LLFile operations.

Attempting to debug an observed LLFile::remove() failure, I was floored to
find that remove() made no attempt whatsoever to report its lack of success!
Add warnif() function to log errno text in platform-dependent way. Support the
notion that for some functions, certain errno values are acceptable -- e.g. we
expect stat() to frequently hit ENOENT -- and need not be logged.
Add commented-out Windows-specific logic to try to provide further information
in the case of EACCES ("Permission denied," e.g. another process has the file
open). To use, enable the code block, download handle.exe and turn on DEBUG
logging for LLFile. handle.exe can be obtained from:
http://technet.microsoft.com/en-us/sysinternals/bb896655