improve feedback if options are missing

[Karl-Philipp Richter]

Running `gcovr` in an arbitrary directory (e.g. `/tmp`) without arguments and options gives feedback

    $ gcovr
    ------------------------------------------------------------------------------
                               GCC Code Coverage Report
    Traceback (most recent call last):
      File "/usr/bin/gcovr", line 1972, in <module>
        print_text_report(covdata)
      File "/usr/bin/gcovr", line 822, in print_text_report
        OUTPUT.write("Directory: "+options.root+"\n")
    TypeError: cannot concatenate 'str' and 'NoneType' objects

It'd be more helpful if a feedback what to specify was provided.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: gcovr 3.2-1
Uname: Linux 4.1.4-040104-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.4
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Sep 9 13:06:26 2015
InstallationDate: Installed on 2015-06-08 (92 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: gcovr
UpgradeStatus: Upgraded to vivid on 2015-06-09 (92 days ago)

[Sylvestre Ledru (sylvestre)]

This is fixed in gcovr 3.4 (uploaded in Debian).