SI-6440 Address regressions around MissingRequirementError

 Go back to using globalError to report when a stub's info is referenced,
 and only throw the MissingRequirementError when compilation really
 must abort due to having a StubTermSymbol in a place where a
 StubClassSymbol would have been a better choice.

 This situation arises when an entire package is missing from the
 classpath, as was the case in the reported bug.

 Adds `StoreReporterDirectTest`, which buffers messages issued
 during compilation for more structured interrogation. Use this
 in two test for manifests -- these tests were using a crude means
 of grepping compiler console output to focus on the relevant output,
 but this approach was insufficient with the new multi-line error
 message emitted as part of this change.

 Also used that base test class to add two new tests: one for
 the reported error (package missing), and another for a simpler
 error (class missing). The latter test shows how stub symbols
 allow code to compile if it doesn't the subset of signatures
 in some type that refer to a missing class.

 Gave the INFO/WARNING/ERROR members of Reporter sensible
 toString implementations; they inherit from Enumeration#Value
 in an unusual manner (why?) that means the built in toString of
 Enumeration printed `Severity@0`.

1 files changed, 4 insertions, 1 deletions

diff --git a/test/files/run/t6440.check b/test/files/run/t6440.check
index b5684daee4..69c253eab4 100644
--- a/test/files/run/t6440.check
+++ b/test/files/run/t6440.check
@@ -1 +1,4 @@
-Stream((), ?)
+pos: source-newSource1,line-9,offset=109 bad symbolic reference. A signature in U.class refers to term pack1
+in package <root> which is not available.
+It may be completely missing from the current classpath, or the version on
+the classpath might be incompatible with the version used when compiling U.class. ERROR