RFR: 8210275: Source Launcher should fail if --source is used without a source file

[Jonathan Gibbons]

Please review a small fix to the source launcher that showed up as
the launcher sometimes ignoring the --source option and finding/running
a class on the class path.

The problem occurs when the classpath is set after the --source option,
causing LM_CLASS mode to override the setting of LM_SOURCE.

In addition, some of the error messages that may be reported by the
source launcher are improved to use "source file" instead of "file" as
an indication that source-file mode is being used.

JBS: https://bugs.openjdk.java.net/browse/JDK-8210275
Webrev: http://cr.openjdk.java.net/~jjg/8210275/webrev.00/index.html

-- Jon