handleLtFiles is ignoring non-NA `err` values returned by ltMergeFiles

[jbrzusto]

So in e.g. MotusDev/Motus-TO-DO#418, a .DTA file with no receiver serno is being silently dropped.

Etiology

• readDTA() is failing with an error which is caught by ltMergeFiles:

  Rscript -e 'library(motusServer);readDTA("ERI_04Nov2018.DTA")'
  ...
  Error in paste0("Lotek-", serno) : object 'serno' not found
  Calls: readDTA -> paste0
  Execution halted

  In the example linked, it appears that the serial number is reported as the Site Code,

  $ grep "Site Code" * | uniq
  ERI_04Nov2018.DTA:Site Code: 8205
  STA_02Dec2018.DTA:Site Code: 6347

  and this is confirmed by a check with the metadatabase:

  `meta -header -html "select * from recvDeps where (serno glob '*8205*' or serno glob '*6347*') and status='active'"

  gives:

  id	serno	receiverType	deviceID	macAddress	status	deployID	name	fixtureType	latitude	longitude	isMobile	tsStart	tsEnd	projectID	elevation
  1321	Lotek-6347	LOTEK	496	0	active	2913	Stangee 2016	Tower	41.61096	-83.23132	0	1457514620.0		64
  1326	Lotek-8205	LOTEK	537	0	active	2917	Erie Marsh 2016	Tower	41.76347	-83.47027	0	1457704830.0		64

i.e. ERI is "Erie" via 8205 and STA is "Stangee" via 6347.

Remedy

• in ltMergeFiles(): if S/N field is blank, treat Site Code as the Lotek serial number iff the filename begins with three letters which match (case insensitively) the first three letters of the name of the active receiver deployment having that serial number, in which case also report a warning
• otherwise, report an error