Update R/makedist.R

Revised two warning messages in makedist.R, both arising in scenarios where user has asked match_on to compute a large number of distances.  They aim to head off two misinterpretations:
1.  either warning might suggest to the user that the matching problem is too large for match_on, as opposed to for fullmatch/pairmatch etc.  Larger distances can take a while to compute, during which the user sits looking at this warning message; she might mistakenly conclude that she's done something to impede match_on, and conclude the software has hit a dead end.  We need to reword to limit this suggestion; here is one possibility.
2. The second of the revised warnings, the warning about large problems that's issued when the user has provided a within argument, may be thrown when the user is moving toward a small enough problem for fullmatch/pairmatch etc, but isn't quite there yet.  Example:

md.hosp <- exactMatch(pr_vcd ~ hosp.code, data=psdata)
md.tmp <- match_on(pr_vcd ~PS.glmer, within=md.hosp, data=pscores)
md.tmp <- match_on(md.tmp, within=caliper(md.tmp, width=.05) )


In these cases it would be nice to avoid suggesting to the user that she's not using match_on correctly, and in general to avoid needlessly alarming her.  Again, open to other ways of doing this.