bitbake: fetch2: Print the complete SRCREV variable name when INVALID

If a particular SRCREV (say for a particular branch) is missing, the
fetcher will currently just report an obtuse error about the "SRCREV"
being invalid. If there is more information is to be had (say from name,
i.e. branch, and pn) then display that as well.

The new error looks something like this:

ERROR: ExpansionError during parsing /home/dvhart/source/poky/meta/recipes-kernel/linux/linux-yocto_3.4.bb: Failure expanding variable do_patch: ExpansionError: Failure expanding variable SRCPV, expression was ${@bb.fetch2.get_srcrev(d)} which triggered exception FetchError: Fetcher failure for URL: 'git://otcgit.jf.intel.com/dvhart/linux-yocto-minnow-3.4.git;protocol=git;nocheckout=1;branch=standard/minnow,meta,emgd-1.14;name=machine,meta,emgd'. Please set SRCREV_emgd_pn-linux-yocto to a valid value

Note the variable listed as invalid is
"SRCREV_emgd_pn-linux-yocto", making it explicit what is wrong.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: bitbake-devel@lists.openembedded.org
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>