Replace sys.exit(-1) with sys.error(msg)

[htmldoug]

Problem

sbt mimaReportBinaryIssues is terminating the sbt process with System.exit(-1).

sbt:lib-weepickle-spices-parsers-play25-v1> mimaReportBinaryIssues
not a directory or jar file: /Users/doug.roper/code/lib-weepickle-spices/parsers-play25/target/scala-2.11/classes
$

Similar error condition to #55, but this PR is about preventing shutdown of the sbt's JVM when a mima problem occurs.

We've forked mima over this issue, but I'd like to get us back on mainline.

Testing

I've verified this manually using a snapshot:

sbt:lib-weepickle-spices> parsers-play25/mimaReportBinaryIssues
[error] stack trace is suppressed; run last parsers-play25 / mimaReportBinaryIssues for the full output
[error] (parsers-play25 / mimaReportBinaryIssues) not a directory or jar file: /Users/doug.roper/code/lib-weepickle-spices/parsers-play25/target/scala-2.11/classes
[error] Total time: 1 s, completed Apr 20, 2020 9:12:06 PM

I didn't add any new scripted tests since the change is pretty straightforward. Existing scripted tests all pass.

cc: @jrudolph @arkban

[dwijnand]

Yep, no problem. Thanks for the PR!

[SethTisue]

relic of MiMa's CLI days, I suppose