Improve Groovysh Error output: For compilation Errors, Users should only be shown actual message

Before:

    groovy:005>     @Override
    ERROR org.codehaus.groovy.control.MultipleCompilationErrorsException:
    startup failed:
    groovysh_parse: 7: unexpected token: @ @ line 7, column 5.
           @Override
           ^

    1 error

            at java_lang_Runnable$run.call (Unknown Source)

After:

    groovy:005>     @Override
    groovysh_parse: 7: unexpected token: @ @ line 7, column 5.
           @Override
       ^
GROOVY-6139: Fix undesirable system.out debug output (introduced with 6139)
GROOVY-7200: Groovysh: Completion for instanceof statements
This commit breaks the API of GroovySyntaxCompletor class

New feature:
groovy:000> x instanceof Gr|<tab>
Now completes
GregorianCalendar  Grab  GrabConfig  GrabExclude ...
GROOVY-7201: Groovysh: Fix Annotations before member methods causing parse error output
Merge pull request #549 from tkruse/groovy7200

Groovysh: GROOVY-7200 + GROOVY-7201
Updated style guide

Extracted heading which was accidentally included in a code example
Merge pull request #550 from dnahodil/master

Updated style guide
GROOVY-7203: don't force lookup object access and fall back to per invocation error instead
GROOVY-7206: NullPointerException when setting an unknown property in a trait