Simpler error messages for missing gradient rules

[mhauru]

Would it be possible to simplify the error messages that Enzyme prints out in cases where there's a library call for which Enzyme has no gradient rule? See e.g.

Current scope:
; Function Attrs: mustprogress willreturn
define "enzyme_type"="{[-1]:Float@double}" double @preprocess_julia_f_8695({} addrspace(10)* noundef nonnull align 16 dereferenceable(40) "enzyme_type"="{[-1]:Pointer, [-1,0]:Pointer, [-1,0,-1]:Float@double, [-1,8]:Integer, [-1,9]:Integer, [-1,10]:Integer, [-1,11]:Integer, [-1,12]:Integer, [-1,13]:Integer, [-1,14]:Integer, [-1,15]:Integer, [-1,16]:Integer, [-1,17]:Integer, [-1,18]:Integer, [-1,19]:Integer, [-1,20]:Integer, [-1,21]:Integer, [-1,22]:Integer, [-1,23]:Integer, [-1,24]:Integer, [-1,25]:Integer, [-1,26]:Integer, [-1,27]:Integer, [-1,28]:Integer, [-1,29]:Integer, [-1,30]:Integer, [-1,31]:Integer, [-1,32]:Integer, [-1,33]:Integer, [-1,34]:Integer, [-1,35]:Integer, [-1,36]:Integer, [-1,37]:Integer, [-1,38]:Integer, [-1,39]:Integer}" "enzymejl_parmtype"="4757723584" "enzymejl_parmtype_ref"="2" %0) local_unnamed_addr #4 !dbg !47 {
top:
  %1 = call {}*** @julia.get_pgcstack() #5
  %ptls_field3 = getelementptr inbounds {}**, {}*** %1, i64 2
  %2 = bitcast {}*** %ptls_field3 to i64***
  %ptls_load45 = load i64**, i64*** %2, align 8, !tbaa !8
  %3 = getelementptr inbounds i64*, i64** %ptls_load45, i64 2
  %safepoint = load i64*, i64** %3, align 8, !tbaa !12
  fence syncscope("singlethread") seq_cst
  call void @julia.safepoint(i64* %safepoint) #5, !dbg !48
  fence syncscope("singlethread") seq_cst
  %4 = addrspacecast {} addrspace(10)* %0 to { i8 addrspace(13)*, i64, i16, i16, i32 } addrspace(11)*, !dbg !49
  %arraylen_ptr = getelementptr inbounds { i8 addrspace(13)*, i64, i16, i16, i32 }, { i8 addrspace(13)*, i64, i16, i16, i32 } addrspace(11)* %4, i64 0, i32 1, !dbg !49
  %arraylen = load i64, i64 addrspace(11)* %arraylen_ptr, align 8, !dbg !49, !tbaa !18, !range !21, !alias.scope !22, !noalias !25
  %inbounds.not = icmp eq i64 %arraylen, 0, !dbg !49
  br i1 %inbounds.not, label %oob, label %idxend, !dbg !49

oob:                                              ; preds = %top
  %errorbox = alloca i64, align 8, !dbg !49
  store i64 1, i64* %errorbox, align 8, !dbg !49, !noalias !50
  %5 = addrspacecast {} addrspace(10)* %0 to {} addrspace(12)*, !dbg !49
  call void @ijl_bounds_error_ints({} addrspace(12)* noundef %5, i64* noundef nonnull align 8 %errorbox, i64 noundef 1) #6, !dbg !49
  unreachable, !dbg !49

idxend:                                           ; preds = %top
  %6 = addrspacecast {} addrspace(10)* %0 to double addrspace(13)* addrspace(11)*, !dbg !49
  %arrayptr6 = load double addrspace(13)*, double addrspace(13)* addrspace(11)* %6, align 16, !dbg !49, !tbaa !33, !alias.scope !53, !noalias !25, !nonnull !7
  %arrayref = load double, double addrspace(13)* %arrayptr6, align 8, !dbg !49, !tbaa !36, !alias.scope !39, !noalias !40
  %7 = call double @dnbeta(double %arrayref, double noundef 1.000000e+00, double noundef 1.000000e+00, double noundef 1.000000e+00, i32 noundef 1) #5, !dbg !54
  ret double %7, !dbg !54
}

No forward mode derivative found for dnbeta
 at context:   %7 = call double @dnbeta(double %arrayref, double noundef 1.000000e+00, double noundef 1.000000e+00, double noundef 1.000000e+00, i32 noundef 1) #5, !dbg !41

Stacktrace:
 [1] nbetalogpdf
   @ ~/.julia/packages/StatsFuns/mrf0e/src/rmath.jl:77
 [2] logpdf
   @ ~/.julia/packages/Distributions/ji8PW/src/univariates.jl:645
 [3] f
   @ ~/projects/Enzyme-mwes/callinst_metadata/noncentralbeta.jl:7

from #1620. For a Julia user who is not used to dealing with any lower level stuff, that's an intimidating looking wall of foreign words and % symbols (note the length of the third line), when the real content seems to "Something ended up calling a C library function double @dnbeta(double %arrayref, double noundef 1.000000e+00, double noundef 1.000000e+00, double noundef 1.000000e+00, i32 noundef 1) and Enzyme doesn't have a forward mode derivative for it."