Print error message when to-be-downloaded file cannot be created

It can happen that the to-be-downloaded file cannot be created in cachedir.
For example, I am an -Sup user, and it is comfortable to set --cachedir to
/mnt/pendrive, which is a FAT filesystem, so files like
capseo-1:0.3-2-i686.pkg.tar.xz cannot be downloaded to there.

Before this patch, pacman didn't give clear output about what happens when
the download code could not create the necessary file. This can be confusing
with -Su. An example output:
***
$ sudo pacman -S capseo bochs --cachedir /c/TEMP

resolving dependencies...
looking for inter-conflicts...

Targets (2): bochs-2.4.6-1  capseo-1:0.3-2

Total Download Size:    0.61 MiB
Total Installed Size:   2.61 MiB

Proceed with installation? [Y/n]
:: Retrieving packages from extra...
warning: failed to retrieve some files from extra
 bochs-2.4.6-1-i686       611.5 KiB   118K/s 00:05 [------------------]  97%
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.
***

After the patch, pacman will give more informative error message (and
pm_errno is set properly):
***
error: could not open file '/c/TEMP/capseo-1:0.3-2-i686.pkg.tar.xz.part': Invalid argument
error: failed to commit transaction (failed to retrieve some files)
***

Unfortunately, the "could not open file" error message is printed for
every mirror (that can be dozens of lines), which is ugly, but at least
informative... Without modifying the download logic (for example, by
introducing -2 return value for _alpm_download() to indicate giving up),
this ugliness cannot be eliminated.

Signed-off-by: Nagy Gabor <ngaba@bibl.u-szeged.hu>
Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/dload.c

@@ -422,6 +422,10 @@ static int curl_download_internal(struct dload_payload *payload,
 	if(localf == NULL) {
 		localf = fopen(payload->tempfile_name, payload->tempfile_openmode);
 		if(localf == NULL) {
+			handle->pm_errno = ALPM_ERR_RETRIEVE;
+			_alpm_log(handle, ALPM_LOG_ERROR,
+					_("could not open file %s: %s\n"),
+					payload->tempfile_name, strerror(errno));
 			goto cleanup;
 		}
 	}