Use DC env variable as the default compiler

Fix #895, so that dub looks for a compiler next to itself first. Also, ...

put some restrictions on the defaultCompiler specified in a DUB
settings.json, such that it must be a compiler name, a path relative
to dub, or an absolute path, including a tilde-prefixed local path.
Unqualified relative paths are no longer allowed from the settings file,
as they make no sense for how dub invokes the compiler.

replace all usages of `exit` ...

- those skipped the ERR trap
- add die function to exit with line number and message

fix failures (due to bash error flags)

unify flags and error handling ...

- also add trap ERR handler to report exact error location

Add test for issue #895.