Avoid calling exit from within a library

[abitmore]

Bug Description
As reported by @nanomobile at #1104 (comment), we have several code that exiting from within a library, which makes them hard to test with boost unit testing framework.

1. https://github.com/bitshares/bitshares-core/blob/master/libraries/app/application.cpp#L425
2. https://github.com/bitshares/bitshares-core/blob/master/libraries/app/application.cpp#L988
3. https://github.com/bitshares/bitshares-core/blob/master/libraries/app/application.cpp#L1019

Libraries should not call exit directly, instead, should call return or throw.

Impacts
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Travis, etc.)
• DEX (the Decentralized EXchange, market engine, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Testing

Steps To Reproduce

• Write a test case which will trigger the exit;
• make and run tests/chain_test;
• tests/chain_test will exit unexpectedly after that test case.

Expected Behavior
tests/chain_test will run all test cases.

CORE TEAM TASK LIST

• Evaluate / Prioritize Bug Report
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation

[nanomobile]

4. https://github.com/bitshares/bitshares-core/blob/master/libraries/egenesis/embed_genesis.cpp#L175
5. https://github.com/troglobit/editline/blob/5be965deec0527b48b393b7aca9cb2a76aec2a99/examples/excallback.c#L109
6. https://github.com/troglobit/editline/blob/5be965deec0527b48b393b7aca9cb2a76aec2a99/examples/excallback.c#L118
7. https://github.com/troglobit/editline/blob/5be965deec0527b48b393b7aca9cb2a76aec2a99/examples/excallback.c#L130
8. https://github.com/troglobit/editline/blob/5be965deec0527b48b393b7aca9cb2a76aec2a99/examples/excallback.c#L150
9. https://github.com/troglobit/editline/blob/5be965deec0527b48b393b7aca9cb2a76aec2a99/examples/excallback.c#L152

I think these are all places in libraries, just trying to find another ones, will write all here if will found any others.

[abitmore]

@nanomobile thanks. The editline examples should be OK.