wire: add error string assertions in tests (google/go-cloud#149)

out.txts that start with "ERROR" can now include substrings to assert
exist in the error messages, one per line.  This gives a very
coarse-grained way of testing for error message quality. It is not a
substitute for manual verification, but gives a "good enough" sanity
check that relevant details are reported.

Updates google/go-cloud#5

internal/wire/testdata/Cycle/out.txt

@@ -1 +1,2 @@
 ERROR
+cycle

internal/wire/testdata/EmptyVar/out.txt

@@ -1 +1,2 @@
 ERROR
+not a provider

internal/wire/testdata/InjectInputConflict/out.txt

@@ -1 +1,3 @@
 ERROR
+foo.Foo
+conflicts with provider

internal/wire/testdata/NoImplicitInterface/out.txt

@@ -1 +1,3 @@
 ERROR
+no provider found
+Fooer

internal/wire/testdata/UnexportedValue/out.txt

@@ -1 +1,2 @@
 ERROR
+unexported identifier privateMsg