Create error message if user provides incorrect item name

[j-harbin]

Right now, in dod.ctd(), the user has the option to provide item="index" to obtain a data frame that contains a list of file names and the time. They can then take the list of file names and do a further step similar to the following:

dod.ctd("BBMP", 2005, "CTD_BCD2005667_1_1_DN.ODF")

We need to print an error message if the user does not provide a correct file name.

[dankelley]

Agreed. Also, pretty soon, we'll want to recode some of this in a function that wraps download.file() in a try block, so we can print nice messages, use destdir and so forth. (PS. dod.ctd() needs a destdir parameter.)

[AnnieHow]

Addressed during zoom with Dan, it's been working since adding dod.download; see below (the ID is wrong)

[j-harbin]

@dankelley, with the screen shot that @AnnieHow provided above, is it okay that we see the red writing before the grey box? I would expect to only see what is shown in the light gray box. If it's okay that the stuff before the grey box is being printed out this issue can be closed again and so can https://github.com/AnnieHow/dod/issues/24

[dankelley]

@j-harbin and maybe @AnnieHow -- feel like a Z to chat about this? I'm avail. (We should not put zoom links here on GH.)

[j-harbin]

Fixed after a Z with AH and DK in commit 29a639e11cc3df97f488d18a74b910d596d382c2 . See screen shot below