As raised in #2383, we currently do a poor job of verifying whether the specified files actually really are easyconfig files before we start processing them...

The fix for this is pretty straightforward: initialize the EasyConfigParser object a bit earlier, since that also includes a "raw" parse of the easyconfig file (basically just interpreting the Python code in it).

This leads to a clear error when something other than an easyconfig file is passed by accident, for example an HTML page obtained by incorrectly downloading an easyconfig file using wget:

$ eb test/framework/sandbox/not_an_easyconfig.eb
== temporary log file in case of crash /tmp/eb-P6752K/easybuild-h_JEyc.log
ERROR: Failed to process easyconfig /Volumes/work/easybuild-framework/test/framework/sandbox/not_an_easyconfig.eb: Parsing easyconfig file failed: invalid syntax (<string>, line 1)