jvm_binary_task: do not insert an error message as the "main" class

Review Request #1590 - Created Jan. 9, 2015 and submitted

Tom Dyas
benjyw, jsirois, stuhood, zundel
If no main class is specified for a JvmBinary target, pants sets
the main class in the jar manifest to the string:

"*** java -jar not supported, please use -cp and pick a main ***"

The goal appears to be to indirectly provide an explanation to a user
who tries to run the jar via `java -jar` why running the jar in that
manner fails (as `java` would print out the failing "class name" which
would be the message string).

This behavior prevents using multiple main classes with Hadoop jars,
however. If a main class is specified in a jar manifest, `hadoop jar`
will use that class to the exclusion of any class name specified on
the `hadoop jar` command line. `hadoop jar` only uses a main class from
the command line if there is no main class specified in the jar manifest.

Jars are perfectly fine without a main class specified in the jar
manifest. Indeed, the docstring states this is the expected behavior.

Used this change to produce a jar file (for Hadoop consumption of course) without a main class defined in the jar manifest.

Stu Hood
John Sirois
John Sirois
Tom Dyas
Review request changed

Status: Closed (submitted)