clarify error message when no software-specific easyblock was found

[boegel]

This changes a very cryptic error message like:

NO LONGER SUPPORTED since v2.0: Fallback to default easyblock ConfigureMake
(from easybuild.easyblocks.generic.configuremake); use "easyblock = \'ConfigureMake\'" in
easyconfig file?;
see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

to:

No software-specific easyblock 'EB_MXNet' found for MXNet

I made a breaking change to the signature of get_easyblock_class for this, basically replacing the default_fallback optional argument with error_on_missing_easyblock.

Should I include a deprecation warning for that, which detects the use of default_fallback and assigns error_on_missing_easyblock instead? Just in case this breaks a script that talks with the framework API?

[wpoely86]

We should probably not break api during minor releases?

[boegel]

@wpoely86 updated, please rereview?

[boegel]

Thanks for the review @wpoely86!