Fix esy libev detection

[anmonteiro]

I'm gonna look separately into getting a test for this as suggested in #680

[aantron]

Could you do one of these two things:

1. Separate the test for the error code into an outer match statement, and raise Failure instead of Not_found, where the payload of Failure will explain what it is that failed, as a form of documentation in the source (even though the message will never be used at runtime).

2. (My preference) Keep this combined match, but change the second case from _, 0 to "false\n", 0, which will make it clear that the opam command doesn't indicate that libev is not installed by returning a non-zero error code, but a non-zero error code means either a more serious failure in opam, or the opam command is missing. In this case, I'd still suggest to change the exception Not_found to Failure with a descriptive payload. I found Not_found to be misleading, because it first reads like the code means that it is specifically libev is not found by opam.

[anmonteiro]

For sure, I'll do 2.

[aantron]

Thank you!