sstate: Show better exception information for failures

Errors like

SState: cannot test file://universal/5f/10/sstate:m4-native:x86_64-linux:1.4.19:r0:x86_64:7:5f108b175274798b17950b5ee686bf61445e90fb204d7f795e5b879b5603e88b_deploy_source_date_epoch.tar.zst

: 'GIT_SSL_CAINFO'

aren't useful, try and improve on this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>