Cannot export to file without extension

[kasperl]

Running fletch export hello.dart to hello leads to an error:

Can't perform 'export' to 'ErrorTarget("Expected 'session(s)', 'class(s)', 'method(s)', 'file(s)', or 'all', but got: \"hello\".")'

[peter-ahe-google]

That's a known limitation for now. But the error message needs to be improved.

[peter-ahe-google]

I'd like to fix the error message in 0.1.0

[peter-ahe-google]

The error message is fixed as part of the fix for #100.

[mit-mit]

This is related to #160