Show helpful error messages when setting line breakpoints

[nlfiedler]

Steve observes that setting a line breakpoint will silently fail to resolve for locations that do not have any code. This was done intentionally to allow for non-Java programming language (NJLP) support. However, Steve suggests the following:

The first thing would be to emulate JDB's behavior and issue a notice
that the breakpoint will be set as soon as the class is loaded. This
would help warn the user if they were expecting the breakpoint to be
set immediately.

Then we could have a heuristic for Java source files, since Java imposes
more stringent rules about class-to-source mapping than most other languages. When the user sets a line breakpoint, we could check the path, and if it ends with ".java", see if we've loaded a class that matches that package. If so, check its locations to see if it includes that line. If it doesn't, issue a warning.