add column to downloads table to store error msg when a download fails
add column to downloads table to store error msg when a download fails

useful for debugging for bugs like bug:3187299
without this error message, one doesn't know WHY downloads
fail.
there is a STOPSHIP comment around storage of this error
message in database.

Change-Id: I1fa49ee9eb97be16aced1611ab30a286b0b6c8d2

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base