implement tryCatch() during processing so app can accommodate partially successful runs

[vpnagraj]

two previous issues have been raised (#82 and #88 ) that speak to the need for LOLAweb to be able to more intelligently check whether or not the processing is working at each "step" (i.e. runLOLA(), aggregateOverGenomeBins(), etc)

right now these checks are if statements that cache a NULL value if the processing functions don't work:

https://github.com/databio/LOLAweb/blob/master/apps/LOLAweb/app.R#L599-L632

@nsheff has pointed out that more robust (and permissive) way to do this checking might be to use tryCatch()