Support for debugging DashR errors needs improvement

[rpkyle]

Currently, DashR lacks adequately informative error messages; errors such as the one seen here are likely to be very hard for developers (and app users) to interpret:

Ideally, we would offer several enhancements to improve the debugging experience when errors are thrown while in debug mode:

• return a full call stack, or a partial one which has been "pruned" of internal function calls either related to the error itself or initial configuration tasks prior to invoking DashR-related functions
• catch the error and instead display it as a warning, which may enable more productive debugging once support for interactive breakpoints (accessed via the callback graph) is implemented; currently an error "freezes" the DashR app

It would also be nice to

• return line numbers and function names for errors; this is complicated by R's current handling of srcref attributes (see below).

https://stat.ethz.ch/R-manual/R-devel/library/base/html/srcfile.html
http://r.789695.n4.nabble.com/Is-there-a-way-to-get-R-script-line-number-td4731127.html
https://journal.r-project.org/archive/2010-2/RJournal_2010-2_Murdoch.pdf

[rpkyle]

The majority of these improvements were made in #87. Line numbers will addressed in an upcoming PR, but for the time being the basic support for improved debugging is now in place.