replace all usages of `exit` ...

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

unify flags and error handling ...

- also add trap ERR handler to report exact error location

better support concurrent fetch of package ...

- other dub might have already released the lock
  leading to another (failing) fetch of a package

mktemp on OSX requires a template

fix #674 - cache needs to be locked for concurrent dub runs ...

- implement lockFile/tryLockFile using mkdir
- use lockFile to synchronize package download