Bad (or inconsistent) error message when compiled file is a directory

[osa1]

$ mkdir test
$ moc test
Is a directory

This is an uncaught OCaml exception printed directly. We should at least show the path that is a directory, otherwise things get difficult to debug in e.g. CI or build scripts.

It's also inconsistent with the error message when e.g. file is not found:

$ moc test
test: No such file or directory

In general it's a good idea to be as helpful as possible and show the problematic path.

I suggest we print

<input> is a directory

or

<input>: Is a directory

to be consistent with the error message when file does not exist.

[osa1]

It seems like you can create a lexer for a non-existent file or a directory and it doesn't fail. It only fails when you ask for the next token. In moc, the line that throws this exception is

motoko/src/pipeline/pipeline.ml

Line 99 in 6d2c020

Parsing.parse (!Flags.error_detail) (parser lexer.Lexing.lex_curr_p) tokenizer lexer

It seems like error handling could be refactored a little bit. Normally user errors are in this format:

<location>: <error kind> <error code>, <error message>

"No such file" and "is a directory" errors don't follow this format.