shared_memory: improve error messages on failure.
shared_memory: improve error messages on failure.

Be careful to leave errno untouched if the file open fails, so that
the latter logging statements that print errno will print the right
message.  Also format the logging text such that the perror-like
output (appending text like ": Permission denied") makes more sense.

Previously we'd end up printing text like
   Try [...] to fix.: Permission denied

BUG=57305
TEST=shared_memory_unittest should still pass

Review URL: http://codereview.chromium.org/3517003

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@60986 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser