alpm: return -1 for error in find_dl_candidates

This is the error value generally used and the calling function
explicitly checks for -1, later causing the error to be missed
and the transaction to continue.

> pacman -S xterm
warning: xterm-369-1 is up to date -- reinstalling
resolving dependencies...
looking for conflicting packages...

Package (1)  Old Version  New Version  Net Change  Download Size

extra/xterm  369-1        369-1          0.00 MiB       0.42 MiB

Total Download Size:   0.42 MiB
Total Installed Size:  1.05 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n]
error: no servers configured for repository: extra
(1/1) checking keys in keyring                                                                 [--------------------------------------------------------] 100%
(1/1) checking package integrity                                                               [--------------------------------------------------------] 100%
error: failed to commit transaction (wrong or NULL argument passed)
Errors occurred, no packages were upgraded.
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