Error handling and stream improvements for command-line apps

[jerinphilip]

1. Avoid copies through stringstream (of widespread use currently in BRT) to read into a string using buffered read.
2. BRT command line does not do error handling - whether the file exists, are we running out of space on write that sort. Handle that as well.

[jerinphilip]

1. Past refactoring PRs have aggressively squeezed stream reads into a single place:

https://github.com/browsermt/bergamot-translator/blob/bcbbfe129525ed2be8dbf00d2da9d412667f1d8d/src/translator/utils.h#L7-L13

2. Having turned off logging by default and seen experienced Marian users use:

// Set marian to throw exceptions instead of std::abort()
marian::setThrowExceptionOnAbort(true);

https://github.com/XapaJIaMnu/translateLocally/blob/6f40bceedb4909dfead81441cfb606582c43dcaf/src/main.cpp#L13

thinking the above would be nice to incorporate.