BLD: output friendlier error messages from scons.

Replace assertion failures with a gentler scons termination.

site_scons/libdiffpybuildutils.py

@@ -8,13 +8,17 @@
 
 MYDIR = os.path.dirname(os.path.abspath(__file__))
 
-GIT_MISSING_ERROR_MSG = """
+EMSG_GIT_MISSING = """
 Cannot determine libdiffpy version.  Compile from a git repository
 or use a source archive from
 
     https://github.com/diffpy/libdiffpy/releases
 """
 
+EMSG_BAD_FALLBACK_VERSION = """
+Inconsistent FALLBACK_VERSION {} vs {} from git tag.
+"""
+
 
 def gitinfo():
     'Extract dictionary of version data from git records.'
@@ -65,12 +69,13 @@ def getversion():
     if gi:
         afb = FALLBACK_VERSION
         gfb = gi['version'].split('.post')[0] + '.post0'
-        emsg = "Inconsistent FALLBACK_VERSION {!r}.  Git tag suggests {!r}."
-        assert gfb == afb, emsg.format(afb, gfb)
+        if gfb != afb:
+            raise RuntimeError(EMSG_BAD_FALLBACK_VERSION.format(afb, gfb))
         rv.update(gi)
     else:
         # Not a git repository.  Require that gitarchive.cfg is expanded.
-        assert '$Format:' not in ga['commit'], GIT_MISSING_ERROR_MSG
+        if '$Format:' in ga['commit']:
+            raise RuntimeError(EMSG_GIT_MISSING)
         rv['commit'] = ga['commit']
         rv['date'] = ga['date']
         # First assume we have an undetermined post-release.  Keep version

src/diffpy/SConscript.version

@@ -5,6 +5,16 @@ import string
 Import('env')
 
 
+def get_version_or_die():
+    from libdiffpybuildutils import getversion
+    try:
+        rv = getversion()
+    except RuntimeError as e:
+        print(e)
+        Exit(1)
+    return rv
+
+
 def parsemajorminor(hcode):
     'Extract major and minor version from a C++ header file.'
     mx = re.search(r'(?m)^#define *DIFFPY_VERSION_MAJOR *(\d+)', hcode)
@@ -69,9 +79,8 @@ vhpp = File('version.hpp')
 if os.path.isfile(str(vhpp.srcnode())):
     majorminor = parsemajorminor(vhpp.srcnode().get_contents())
 else:
-    from libdiffpybuildutils import getversion
     vtpl = File('version.tpl')
-    gver = getversion()
+    gver = get_version_or_die()
     vhpp, = env.BuildVersionCode(['version.hpp'], vtpl)
     env.Depends(vhpp, env.Value(gver['version'] + gver['commit']))
     majorminor = (gver['major'], gver['minor'])