Including a nonexistent portgroup should be an error

[ryandesign (Ryan Carsten Schmidt)]

When a Portfile includes a portgroup that does not exist, MacPorts prints a warning, then tries to build the port anyway, which can't possibly succeed. For example, I saw this when trying to use an old version of a portfile I grabbed from our history:

Warning: PortGroup python27 1.0 could not be located. python27-1.0.tcl does not exist.
Warning: PortGroup python27 1.0 could not be located. python27-1.0.tcl does not exist.
--->  Computing dependencies for py27-pylibelf
--->  Fetching distfiles for py27-pylibelf
--->  Verifying checksums for py27-pylibelf
--->  Extracting py27-pylibelf
--->  Applying patches to py27-pylibelf
--->  Configuring py27-pylibelf
Error: Failed to configure py27-pylibelf: configure failure: command execution failed

Including a nonexistent portgroup should be an error after which we stop, not a warning after which we try to proceed.

[kurthindenburg (Kurt Hindenburg)]

​https://github.com/macports/macports-base/pull/169

[kurthindenburg (Kurt Hindenburg)]

[d9139328fa414edaa29d993bc1801dea904f3f10/macports-base]