matrices: Add message to exception
matrices: Add message to exception

Raising an empty exception confuses the IGNORE_EXCEPTION_DETAIL flag
when running doctests under Python 3. Add a simple string ("Matrices
size mismatch.") to the raised exception and the doctest.

hargup/sympy,liangjiaxing/sympy,maniteja123/sympy,toolforger/sympy,pandeyadarsh/sympy,lidavidm/sympy,cccfran/sympy,ga7g08/sympy,shikil/sympy,sunny94/temp,Curious72/sympy,AunShiLord/sympy,hargup/sympy,grevutiu-gabriel/sympy,kevalds51/sympy,Designist/sympy,beni55/sympy,sahmed95/sympy,Vishluck/sympy,Titan-C/sympy,ChristinaZografou/sympy,AkademieOlympia/sympy,MechCoder/sympy,kaichogami/sympy,MridulS/sympy,maniteja123/sympy,hrashk/sympy,kumarkrishna/sympy,Davidjohnwilson/sympy,jamesblunt/sympy,emon10005/sympy,ChristinaZografou/sympy,lindsayad/sympy,madan96/sympy,drufat/sympy,meghana1995/sympy,madan96/sympy,asm666/sympy,Vishluck/sympy,pbrady/sympy,chaffra/sympy,wyom/sympy,kevalds51/sympy,Sumith1896/sympy,saurabhjn76/sympy,yukoba/sympy,kaichogami/sympy,sahilshekhawat/sympy,abhiii5459/sympy,cswiercz/sympy,yashsharan/sympy,chaffra/sympy,Mitchkoens/sympy,ga7g08/sympy,atsao72/sympy,sampadsaha5/sympy,maniteja123/sympy,sahmed95/sympy,vipulroxx/sympy,ahhda/sympy,sahilshekhawat/sympy,AunShiLord/sympy,AkademieOlympia/sympy,VaibhavAgarwalVA/sympy,rahuldan/sympy,iamutkarshtiwari/sympy,rahuldan/sympy,wyom/sympy,sampadsaha5/sympy,shipci/sympy,mcdaniel67/sympy,sampadsaha5/sympy,garvitr/sympy,srjoglekar246/sympy,pbrady/sympy,kevalds51/sympy,Designist/sympy,kmacinnis/sympy,Designist/sympy,jbbskinny/sympy,Davidjohnwilson/sympy,wanglongqi/sympy,emon10005/sympy,Gadal/sympy,Arafatk/sympy,farhaanbukhsh/sympy,Gadal/sympy,atreyv/sympy,ga7g08/sympy,garvitr/sympy,oliverlee/sympy,amitjamadagni/sympy,abloomston/sympy,jamesblunt/sympy,kaushik94/sympy,garvitr/sympy,kaichogami/sympy,debugger22/sympy,amitjamadagni/sympy,yashsharan/sympy,AkademieOlympia/sympy,yashsharan/sympy,kmacinnis/sympy,jerli/sympy,dqnykamp/sympy,Davidjohnwilson/sympy,drufat/sympy,sahmed95/sympy,ChristinaZografou/sympy,Mitchkoens/sympy,abloomston/sympy,hrashk/sympy,kmacinnis/sympy,pandeyadarsh/sympy,grevutiu-gabriel/sympy,oliverlee/sympy,MechCoder/sympy,AunShiLord/sympy,jbbskinny/sympy,yukoba/sympy,shipci/sympy,farhaanbukhsh/sympy,Mitchkoens/sympy,asm666/sympy,skidzo/sympy,skidzo/sympy,shipci/sympy,kaushik94/sympy,sunny94/temp,jamesblunt/sympy,vipulroxx/sympy,Curious72/sympy,debugger22/sympy,iamutkarshtiwari/sympy,Sumith1896/sympy,wanglongqi/sympy,oliverlee/sympy,Curious72/sympy,shikil/sympy,lindsayad/sympy,hrashk/sympy,skirpichev/omg,VaibhavAgarwalVA/sympy,beni55/sympy,moble/sympy,MridulS/sympy,aktech/sympy,postvakje/sympy,cccfran/sympy,moble/sympy,yukoba/sympy,abloomston/sympy,mcdaniel67/sympy,moble/sympy,cswiercz/sympy,Titan-C/sympy,Arafatk/sympy,iamutkarshtiwari/sympy,flacjacket/sympy,Gadal/sympy,chaffra/sympy,abhiii5459/sympy,jaimahajan1997/sympy,atsao72/sympy,wyom/sympy,farhaanbukhsh/sympy,saurabhjn76/sympy,saurabhjn76/sympy,dqnykamp/sympy,kumarkrishna/sympy,hargup/sympy,beni55/sympy,cswiercz/sympy,pandeyadarsh/sympy,Shaswat27/sympy,Shaswat27/sympy,sahilshekhawat/sympy,sunny94/temp,atreyv/sympy,shikil/sympy,emon10005/sympy,souravsingh/sympy,postvakje/sympy,liangjiaxing/sympy,pbrady/sympy,Titan-C/sympy,toolforger/sympy,meghana1995/sympy,kaushik94/sympy,lidavidm/sympy,Shaswat27/sympy,aktech/sympy,mafiya69/sympy,grevutiu-gabriel/sympy,diofant/diofant,bukzor/sympy,Sumith1896/sympy,drufat/sympy,jbbskinny/sympy,Vishluck/sympy,wanglongqi/sympy,meghana1995/sympy,aktech/sympy,jaimahajan1997/sympy,mafiya69/sympy,dqnykamp/sympy,jerli/sympy,skidzo/sympy,ahhda/sympy,lidavidm/sympy,bukzor/sympy,vipulroxx/sympy,postvakje/sympy,atsao72/sympy,MechCoder/sympy,lindsayad/sympy,mcdaniel67/sympy,rahuldan/sympy,kumarkrishna/sympy,jerli/sympy,cccfran/sympy,jaimahajan1997/sympy,liangjiaxing/sympy,atreyv/sympy,MridulS/sympy,bukzor/sympy,souravsingh/sympy,mafiya69/sympy,VaibhavAgarwalVA/sympy,asm666/sympy,toolforger/sympy,souravsingh/sympy,abhiii5459/sympy,Arafatk/sympy,debugger22/sympy,madan96/sympy,ahhda/sympy