Improves error handling in c sass_interface.cpp

Handle all exceptions and do not let them propagate to
any underlying C code, as it will terminate the program.
Changed some messages and made all starting with uppercase.
Changed "source string" indicator to "stdin".
Allows C bindings to overwrite input_path on sass_compile

Fixes a bug when entry_point is implemented by C bindings
for string compilations (libsass tries to read that file).
Fixes some clang warnings
Merge pull request #496 from mgreter/feature/better-error-handling

Improves error handling for C Binding