Detect a missing javac and provide a slightly more helpful error message.
Detect a missing javac and provide a slightly more helpful error message.

Note the hyphens in the error message are required because the output of
this scripts is used directly in a Makefile target.

Signed-off-by: Rod Whitby <f9f03903429036ef609ed7ca822fddfa76155c29@whitby.id.au>

RR-msm7x30/platform_manifest,xorware/android_build,fallouttester/platform_manifest,knone1/platform_manifest,ND-3500/platform_manifest,hagar006/platform_manifest,see4ri/lolili,xorware/android_build,pranav01/platform_manifest,xorware/android_build,Hybrid-Power/startsync,xorware/android_build,xorware/android_build