Cleanup asserts & throws - replace with invariants

[sonodtt]

Commit replaces asserts & throws in string_abstraction.cpp only - and replaces with appropriate invariants and exceptions.

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

A non-empty commit message body would be nice, but otherwise looks good to me. One day we should add tests for this code...

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 165e0e6).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85586480