typeck::check::_match: Better error handling
typeck::check::_match: Better error handling

Previously this was an Option::unwrap() which failed for me.
Unfortunately I've since inadvertently worked around the bug and have
been unable to reproduce it. With this patch hopefully the next person
to encounter this will be in a slightly better position to debug it.

aepsil0n/rust,andars/rust,quornian/rust,TheNeikos/rust,mahkoh/rust,gifnksm/rust,aneeshusa/rust,jroesch/rust,dwillmer/rust,l0kod/rust,sae-bom/rust,zaeleus/rust,rprichard/rust,pelmers/rust,nwin/rust,jroesch/rust,philyoon/rust,andars/rust,dwillmer/rust,l0kod/rust,aidancully/rust,ktossell/rust,zachwick/rust,zaeleus/rust,aidancully/rust,mahkoh/rust,ktossell/rust,untitaker/rust,omasanori/rust,ruud-v-a/rust,jbclements/rust,vhbit/rust,mihneadb/rust,kwantam/rust,gifnksm/rust,rohitjoshi/rust,kwantam/rust,pelmers/rust,bombless/rust,aneeshusa/rust,aepsil0n/rust,krzysz00/rust,kimroen/rust,dwillmer/rust,rohitjoshi/rust,barosl/rust,ejjeong/rust,philyoon/rust,XMPPwocky/rust,barosl/rust,l0kod/rust,ejjeong/rust,pshc/rust,avdi/rust,cllns/rust,0x73/rust,jbclements/rust,ejjeong/rust,GBGamer/rust,XMPPwocky/rust,mahkoh/rust,AerialX/rust-rt-minimal,aneeshusa/rust,gifnksm/rust,emk/rust,zubron/rust,pczarn/rust,bombless/rust,zubron/rust,untitaker/rust,0x73/rust,untitaker/rust,vhbit/rust,kimroen/rust,kimroen/rust,vhbit/rust,GBGamer/rust,ejjeong/rust,nwin/rust,rprichard/rust,AerialX/rust,erickt/rust,avdi/rust,untitaker/rust,l0kod/rust,quornian/rust,zachwick/rust,ktossell/rust,mahkoh/rust,ruud-v-a/rust,seanrivera/rust,miniupnp/rust,victorvde/rust,nham/rust,reem/rust,krzysz00/rust,cllns/rust,richo/rust,dinfuehr/rust,graydon/rust,quornian/rust,nwin/rust,untitaker/rust,richo/rust,barosl/rust,philyoon/rust,ebfull/rust,AerialX/rust-rt-minimal,seanrivera/rust,TheNeikos/rust,pelmers/rust,mihneadb/rust,KokaKiwi/rust,barosl/rust,jashank/rust,sae-bom/rust,jbclements/rust,erickt/rust,carols10cents/rust,hauleth/rust,quornian/rust,erickt/rust,KokaKiwi/rust,jashank/rust,rohitjoshi/rust,l0kod/rust,l0kod/rust,pelmers/rust,TheNeikos/rust,seanrivera/rust,AerialX/rust-rt-minimal,carols10cents/rust,carols10cents/rust,GBGamer/rust,hauleth/rust,mihneadb/rust,zachwick/rust,pczarn/rust,P1start/rust,philyoon/rust,ebfull/rust,rprichard/rust,mdinger/rust,emk/rust,dwillmer/rust,bombless/rust,pczarn/rust,aneeshusa/rust,rohitjoshi/rust,hauleth/rust,ktossell/rust,victorvde/rust,zaeleus/rust,nwin/rust,pshc/rust,miniupnp/rust,jashank/rust,zubron/rust,pshc/rust,TheNeikos/rust,cllns/rust,graydon/rust,barosl/rust,mdinger/rust,erickt/rust,ktossell/rust,kwantam/rust,AerialX/rust-rt-minimal,AerialX/rust-rt-minimal,mihneadb/rust,zaeleus/rust,jashank/rust,barosl/rust,AerialX/rust,P1start/rust,victorvde/rust,richo/rust,philyoon/rust,ebfull/rust,0x73/rust,kimroen/rust,nwin/rust,emk/rust,sae-bom/rust,jbclements/rust,pczarn/rust,rohitjoshi/rust,erickt/rust,vhbit/rust,aepsil0n/rust,miniupnp/rust,robertg/rust,kwantam/rust,pshc/rust,zachwick/rust,vhbit/rust,AerialX/rust,0x73/rust,kimroen/rust,jashank/rust,nwin/rust,jroesch/rust,mahkoh/rust,krzysz00/rust,mdinger/rust,andars/rust,richo/rust,ruud-v-a/rust,andars/rust,dinfuehr/rust,emk/rust,nham/rust,graydon/rust,robertg/rust,reem/rust,robertg/rust,jbclements/rust,hauleth/rust,ktossell/rust,bombless/rust-docs-chinese,richo/rust,avdi/rust,kimroen/rust,aepsil0n/rust,philyoon/rust,pelmers/rust,jbclements/rust,P1start/rust,miniupnp/rust,AerialX/rust,nwin/rust,AerialX/rust,vhbit/rust,defuz/rust,defuz/rust,jbclements/rust,zubron/rust,ebfull/rust,zaeleus/rust,robertg/rust,emk/rust,XMPPwocky/rust,robertg/rust,jbclements/rust,avdi/rust,omasanori/rust,jroesch/rust,ebfull/rust,AerialX/rust,TheNeikos/rust,seanrivera/rust,avdi/rust,quornian/rust,kwantam/rust,zachwick/rust,victorvde/rust,zachwick/rust,0x73/rust,robertg/rust,vhbit/rust,pshc/rust,zubron/rust,andars/rust,P1start/rust,KokaKiwi/rust,reem/rust,jashank/rust,ruud-v-a/rust,bombless/rust,mvdnes/rust,miniupnp/rust,aidancully/rust,avdi/rust,jroesch/rust,nham/rust,TheNeikos/rust,rohitjoshi/rust,dinfuehr/rust,bombless/rust,dwillmer/rust,zubron/rust,sae-bom/rust,gifnksm/rust,carols10cents/rust,seanrivera/rust,omasanori/rust,jroesch/rust,barosl/rust,aepsil0n/rust,mdinger/rust,graydon/rust,nham/rust,seanrivera/rust,pczarn/rust,cllns/rust,aidancully/rust,kwantam/rust,jroesch/rust,nham/rust,pshc/rust,quornian/rust,mahkoh/rust,carols10cents/rust,defuz/rust,GBGamer/rust,reem/rust,defuz/rust,quornian/rust,KokaKiwi/rust,krzysz00/rust,aepsil0n/rust,erickt/rust,hauleth/rust,graydon/rust,gifnksm/rust,dwillmer/rust,ktossell/rust,XMPPwocky/rust,nham/rust,aidancully/rust,0x73/rust,XMPPwocky/rust,dinfuehr/rust,mihneadb/rust,bombless/rust,vhbit/rust,P1start/rust,emk/rust,pczarn/rust,jbclements/rust,dinfuehr/rust,zubron/rust,ejjeong/rust,pshc/rust,pshc/rust,defuz/rust,aneeshusa/rust,rprichard/rust,hauleth/rust,nham/rust,mihneadb/rust,andars/rust,pczarn/rust,rprichard/rust,KokaKiwi/rust,aidancully/rust,krzysz00/rust,cllns/rust,sae-bom/rust,miniupnp/rust,mvdnes/rust,ruud-v-a/rust,KokaKiwi/rust,victorvde/rust,krzysz00/rust,richo/rust,l0kod/rust,carols10cents/rust,gifnksm/rust,GBGamer/rust,pelmers/rust,emk/rust,aneeshusa/rust,omasanori/rust,reem/rust,victorvde/rust,miniupnp/rust,zubron/rust,mvdnes/rust,zaeleus/rust,omasanori/rust,mdinger/rust,defuz/rust,jashank/rust,P1start/rust,dwillmer/rust,GBGamer/rust,nwin/rust,unti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