throw RuntimeError if no system is found by expand_sys_str

[njzjz]

Fix #1286.

[codecov]

Codecov Report

Patch coverage: 50.00% and no project coverage change.

Comparison is base (a356e70) 47.79% compared to head (5d56ad0) 47.79%.
Report is 2 commits behind head on devel.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #1292   +/-   ##
=======================================
  Coverage   47.79%   47.79%           
=======================================
  Files          82       82           
  Lines       14586    14588    +2     
=======================================
+ Hits         6972     6973    +1     
- Misses       7614     7615    +1     

Files Changed	Coverage Δ
dpgen/util.py	93.68% <50.00%> (-0.94%)	⬇️

... and 2 files with indirect coverage changes

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