When running with a timeout, pyomo is expecting the glpk (for example) to terminate itself. It allows 1 second for this or 1% of the timeout, whichever is longer. If the machine is stressed though then this allowance can be exceeded. The resulting error is misleading:

Solver (glpk) returned non-zero return code (-1)
Solver (glpk) did not exit normally

The return code though comes from pyomo itself and is what pyomo generates when it kills glpk. Positive return codes come from glpk etc and should be reported in this way. Negative return codes which come from pyomo should come with an appropriate message such as for -1:

Solver (glpk) killed after timeout allowance exceeded

and for -2 (from my memory):

Solver (glpk) cannot be found