additional error handling when checking for file existence

[veggiesaurus]

Closes #945 by adding additional error handling when calling fs::exists

[jolopezl]

I have a few comments. A couple of minor comments, and another more general across the changes regarding the use of fs::exists. Is the purpose of using fs::exists(p, ec) just using the overload of fs::exists that doesn't throw an exception?

[confluence]

I agree that we should be consistent about either using the functions that throw or the functions that do not.

[veggiesaurus]

I agree that we should be consistent about either using the functions that throw or the functions that do not.

I have changed them all, except for one: SimpleFrontendServer::ClearObjectFromString has calls in a try-catch and we actually want to get hold of the exception information in this case, so I've left it is.

[jolopezl]

Thanks for addressing the comments!. Looks great.