error() functions does not work correctly

[GiovanniBussi]

This was reported by @ollyfutur . The Action::error function results in a segmentation fault when called within the calculate function. A way to reproduce it is to go in plumed2/regtest/basic/rt-make-load and test these changes in the file Distance10.cpp

• First, use error("xxxx") in the constructor. This will just make the test file and result in a tmp/log_threads10 containing the following line:

PLUMED: ERROR in input to action DISTANCE with label d_loaded : xxxx

• Then, use error("xxxx")in the calculate function. The code will crash with abus error`.

• Then, use plumed_error()<<"xxxx"; in the calculate function. The stderr will contain something like this:

libc++abi: terminating due to uncaught exception of type PLMD::Plumed::ExceptionError: 
(Distance10.ziScdp.cpp:87) virtual void PLMD::Distance::calculate()
xxxx

@ollyfutur suggested that the error happens at if( !keywords.exists("NO_ACTION_LOG") ). @gtribello does this make sense?